Maybe the word " snoop " could be a good synonym.
I caught him snooping around in my office.
Reporters were snooping on the celebrities
english.stackexchange.com/users/3782/Manoochehr
You can also use "notice", "observe", "monitor", "track", "keep an eye on", "keep track of", "keep tabs on" -- all these have slightly different meanings, depending on what you mean to convey when you'd use spy .
Use notice if you mean to say that you suddenly noticed a detail of some sort.
Use observe if you mean to say that you're dispassionately watching something unfold, or noticing a detail or series of details with emotional detachment.
Use monitor if you mean to say that you're continually watching something with some measurement in mind that would prompt you to step in or take action if the measurement goes outside a certain range.
All of the keep... phrases are informal ways of saying that you're going to spend some casual effort going forward to (essentially) monitor something -- they're mostly casual ways of using the monitor sense of spy .
Notice that, strictly speaking, spy can carry a connotation of secret, covert behaviour.
When you spy you have an agenda of some sort, or a secret purpose.
The most neutral version of spy is to suddenly notice something: the longer the activity spent observing, the more you convey a sense of secretness.
english.stackexchange.com/users/4747/Viktor Haag
To spy , in the sense of watching someone or something, could also mean keeping that thing under surveillance.
In that sense, spy (the verb) has approximately the same meaning as surveil .
english.stackexchange.com/users/588/Ed Guiness
If you mean to spy then you can use snoop (informal), keep under surveillance/observation , keep a watch on , keep an eye on , observe , spot .
If you mean spy used as noun, then you can use secret agent , undercover agent , intelligence agent , double agent , mole , snoop .
Which word/phrase suits better depends on the context.
english.stackexchange.com/users/252/kiamlaluno
infiltrator, undercover agent
english.stackexchange.com/users/None/
This depends on context and connotation.
There are spies-heroes and there are sneaks, sneakers, snitches, informers, finks, noses, squeakeres, denunciators, questmans, talebearers, whistlers, grass, nark, noser, stool pigeon, checker, squeal, zuch, rat, rounder, delator, etc.
Seems like negative characters are always have better and more specific characteristic terms than positive ones.
english.stackexchange.com/users/3085/Gennady Vanin Геннадий Ванин
I know the word "watch" is used as a synonym of spy. Are there other synonyms that are used by native speakers?
What is the synonym of "spy"?
english.stackexchange.com
<word-choice><synonyms>
english.stackexchange.com/questions/12066
english.stackexchange.com/users/3997/Ehsan
The exact term you might want to use will depend on context and connotation. Other potential synonyms for the word "spy" include "snoop", "observe" "keep an eye on", although all of these have slightly different meanings depending on the context. The spy, in the sense of watching soneone, could also mean keeping that person under surveillance. There are numerous synonyms for "spy" in terms of a verb,  but a "spy" is also a noun.  Other nouns are  "infiltrator" "undercover agent". Synonym verbs for "to spy" are "to snoop", with more a connotation of sneakily checking out things  unbeknownst to others. There are also a wide range of verbs like "notice", "observe", "monitor", "track", "keep an eye on", "keep track of", "keep tabs on" -- all these have slightly different meanings, depending on what you mean to convey when using "to spy"- each one is used in specific contexts. "Surveill" can also be used instead of "to spy".  The context will dictate the choice of words and how the individual or action is depicted.
The exact term you might want to use will depend on context and connotation. Other potential synonyms for the word "spy" include "snoop", "observe" "keep an eye on", although all of these have slightly different meanings depending on the context. The spy, in the sense of watching soneone, could also mean keeping that person under surveillance. Snoop' or 'sneak' could be used to indicate surreptitious spying on someone. On the other hand, 'notice', 'observe', 'monitor', 'track', 'keep an eye on', 'keep track of', 'keep tabs on', 'keep under surveillance/observation' or 'keep a watch on' have the connotation of more official monitoring.
Synonyms for spy include, snoop, notice, observe, monitor, spot track, keep an eye or on, keep track of, keep tabs on, keep under surveillance/observation. Spy used as noun, synonyms include; secret agent, infiltrator, undercover agent,  intelligence agent,  double agent,  mole. The answer to this question will depend on the context and whether "spy" is used as a noun or a verb. If "spy" is a verb, then words such as "snoop", "track" or "serveil" may be suitable substitutes. If, on the other hand, "spy" is used as a noun, "infiltrator" or "undercover agent" may be acceptable.
"notice", "observe", "monitor", "track", "keep an eye on", "keep track of", "keep tabs on" -- all these have slightly different meanings, depending on what you mean to convey when you'd use spy . The noun "spy" as infiltrator, underover agent
"notice", "observe", "monitor", "track", "keep an eye on", "keep track of", "keep tabs on" -- all these have slightly different meanings, depending on what you mean to convey when you'd use spy . The noun "spy" as infiltrator, underover agent
If you mean spy used as noun, then you can use secret agent , undercover agent , intelligence agent , double agen , mole , snoop .
Snoop as a potential synonym as in "snooping around the office" Numerous verb synonyms. Spying as surveillance ;watching someone from a distance Context will determine the word. The noun for spy is "infiltrator", "undercover agent"
Snoop' or 'sneak' could be used to indicate surreptitious spying on someone. Notice', 'observe', 'monitor', 'track', 'keep an eye on', 'keep track of', 'keep tabs on', 'keep under surveillance/observation' or 'keep a watch on' have the connotation of more official monitoring.
The answer to this question will depend on the context and connotation. Words such as "snoop", "track" or "serveil" may serve as a good substitute for "spy" as a verb. "Infiltrator" or "undercover agent" may be a suitable substitute for "spy" as a noun.