Question: A PC in my campaign that is tailing an NPC. She is basically posing as one of his body guards.

At some point I want him to suspect that maybe something is up, and I'm thinking she would either roll a deception check or a stealth check. I just don't know which.

Answer: 
### Posing as someone else typically uses Deception checks.

In the situation you describe, the PC is posing as a bodyguard and doesn't want to be exposed. Whether this means maintaining a disguise, or hiding her motives, or keeping up a convincing guard persona, the [Charisma (Deception)](https://roll20.net/compendium/dnd5e/Ability%20Scores#toc_33) skill is probably most appropriate.

> 
> **Deception**. Your Charisma (Deception) check determines whether you can convincingly hide the truth, either verbally or through your actions. This Deception can encompass everything from misleading others through ambiguity to telling outright lies. Typical situations include trying to fast- talk a guard, con a merchant, earn money through gambling, pass yourself off in a disguise, dull someone’s suspicions with false assurances, or maintain a straight face while telling a blatant lie.
> 
> 
> 

To determine the PC's success at posing as a guard while the NPC is observing her, you would probably have the PC and NPC roll contested skill checks. Typically the NPC would roll a Wisdom (Insight) check versus the PC's Charisma (Deception) check, although as the DM you have the liberty to choose which skill the NPC rolls, depending on the circumstances.
It's a fine line, but it comes down to "Is the player trying to be unseen right now?" or "Is the player trying to fool someone into thinking they're a guard right now?"

If the player's trying to sneak into the guard detail unseen, it's a stealth check against perception. If she is showing up and saluting and pretending to be a guard, it's a deception check against insight. If the NPC suspects something, the player can attempt to hide (stealth), or convince the NPC she's really a guard (deception)

Also keep in mind the difference between passive stats and checks. 
-The guards will be actively looking for trouble, but the NPC will probably be distracted with business or his own thoughts and only passively paying attention. 
-A guard who is keeping file and chatting with his neighbor may not actively pay attention to how she's marching or who she claims to be, but guard captain who's suspicious of her will be actively looking for incorrect information when she talks and studying her appearance.