I generally don't care enough about other people's motivations to really try to figure them out, I suppose though body language, tone, inflection, word choice, are all good indicators, though generally easy to misconstrue especially if you are lacking practice/reference.
Most people generally only have shallow motivations anyway, they might say what they are saying to be friendly, trick you, whatever, but once it is over they are on to the next thing, and only you can control how you felt about it.
It is easy to get wrapped up in hidden motivations if that is what you are looking for, doesn't mean to accept people at face value, but just that at the end of the day don't waste your life worrying about agendas, especially if you are easily distressed by them. You can only focus on so much at once, try to take everything in and your mind just becomes a mess.
Back of the mind, not the front. Aware but not controlled by. Do the best you can to deal day by day, watch how others interact and try to muster up enough energy to put on a similar front. May be repressive, but sometimes it might be necessary to attempt to focus on the here and now and ignore your preferences. Harder for some, I personally have always hated it and have never been good at keeping it up for long, but most interaction is inherently shallow. It's sad.