Go with recommendation by friends whom engaged them before, and listen to their feedbacks.
Also different PI work different way. So you need to see which way is more preferred for you.
1) A PI may follow the person for continuous 7-10 days.
2) A PI will only follow the person upon 'activation' by you.
#1 is good if you know that's he having affair or what and is continuously going for it OR to lay a 'trap' for him...
#2 is good if you doesn't know when he's finding the affair... But he will msg/call you that he have appointment or meeting friends for meal etc...