Right mindfulness

When I'm practising I've noticed that a thought arises but I am noticing this after the fact. With a sensation or sound I can notice it in that moment but thought seems to always be in the past. What is the correct way to be mindful of this? Do I label the thought for what it was such as imagining, planning, worrying etc or do I notice that I'm remembering a past event and label what is happening as remembering?