You've already stated that there is no stealth in space. Good. Anyone who wants to counter that can find an answer on this question explaining that no <PERSON>, there is no stealth in space.
So rather than to answer your question of how to detect another ship in an empty system, lets ask how you remain hidden in such a scenario. We've already determined that it's not possible, so lets ask that question anyway.
Answer: don't point your drive at the person you're hiding from, or if you do, make sure there's something else between you and them.
<PERSON>. Now you know how to detect another ship: its drive is pointed at you (or at least, in your vague general direction) and there isn't anything between you and them. How visible is it? Well, there's a nice link from the above question that states that "a single attitude control thruster of the Space Shuttle can be detected at 15 million km range and using main engines it can be detected from Uranus. With current technology."
Mind, current technology scans the sky very slowly, but pointed in the right direction it'd know. Oh it would know you were there. 199
And how fast you were going, in what direction, and how much acceleration you had (and in what direction), and could predict your course through the solar system (to within some degree of accuracy, not due to the ship or even to the light-time delay, but rather due to the N-Body Problem). Beef up the detection tech in your story so it's got the same resolution as current tech, only it scans faster, and ta da.
The TV show The Expanse has done an amazing job of being Hard Sciency about space travel and communication and detection thereof. The only unreal thing about it is the engines, capable of providing upwards of 10G worth of thrust on virtually no fuel (which, if we're being frank, is the major limiting factor to our civilization's colonization of the other planets). Oh, and there's the...
! aliens and the alien tech that does apparent magic.
But it's not terribly relevant to this answer.
Every instance of stealth in the show has been done under a hard-science approach: decoys ("this isn't the ship you're looking for, sorry!"), flak clouds ("sure, you see us, but can you pick out which 'us' is the right one amidst this cloud of radar jamming junk?"), or pure gravitational assisted flight (if no one knows to look for you and you don't fire up the GIANT GLOWING TORCH BEACON OF LOOK AT ME I'M MISTER MEESEEKS then you can drift around the system at your leisure and be impossible to spot). The gravitational stealth approach in the show had a 45 second slice that broke my suspension of disbelief, as <PERSON> came around the curve of a moon, spotted a Martian vessel and was able to reverse course without being spotted. He was still using the attitude control thrusters, but they appear in the show as gas-vent propulsion, not torches, but the part that broke it for me was that he was able to abort his trajectory enough to slip back around the horizon. Honestly they should have omitted that scene. The quip about "oh yeah, the moons hide them from us too" is clever, but that's the only purposed it served. 947