It's definivly hard to know if a GPS tracker is used unless you look for it and finally find it.
So it could either be in the vehicle, or outside. You can find guide on the net on where to look for. A RF detector might help you locate it more easily eventually.
A GPS tracker is not something that they might want to use, especially if you suspect being followed, because if you find the tracker, they can be in legal trouble, unless they don't care about it or are from the law enforcement themself.
A sign that a GPS is used is if you are not followed, but you end up at your destination with your stalker. But it's not that simple.
It could also be that they already followed you for some time and know your habits. So now, it is just a matter for them of making a phone call, and one people shows up at your rendezvous point.
It could also be that they use more than one vehicle to track you, and even people on foot (I call them "spotters").
It goes like this:
People acting casual on the street can inform people in vehicle the direction you are heading and begin the tail far from your home. And you will never suspect any vehicle is tailing you. It could even be in front of you instead of being behind you.
You could try making right and left turn, but if they use multiple vehicle, the one in top in the queue will continue its route straight ahead while some other vehicle behind it will eventually turn and follow you. You might not be able to see it, until they catch up with you.
You might eventually find clues if they make mistakes or you are extremely watchfull.
You can watch vehicle for their path and try to notice something unusual. Like U-turn, crossing a vehicle path and then get it to follow you and so on.
If this is too much job to look for other vehicles while you drive, you can use dashcam, but this will mean minutes if not hours of video to review afterwards.
A dashcam at the rear might be the most usefull, but not the most easy to find and to install. I had some success with a simple raspberry pi and its camera, hooked up to an external power bank. This is cheap and removable.
As a conclusion, you might want to think of why are you are such a target for those people, given you are actually followed.
May be it's just a mind game after all. Just to harrass you and make you feel followed, make you feel you have no privacy, that you can't hide from them.
In this case, they will try to show up and let you know by whatever mean that you have been followed, arriving at your destination, or even afterwards.
So, keep calm, and keep your eyes peeled but don't over do it otherwise you really turn paranoid, and that's they might want after all.