Applying DFS algorithm to a transition system to find reachable states

Question

Currently working on a past exam question which tells me to compute the product of two transition systems and then use DFS to find the reachable states of the product. I learnt how to compute the product of transition systems by looking at pages 43 and 44 in the "Principles of Model Checking" book by Christel Baier and Joost-Pieter Katoen (if anyone has a copy).

I understand that the way they found the reachable states in page 44 is by tracking the value of the variable y for each state and seeing if it holds, but in my particular question I'm not using a program graph; it's a transition system that uses only sigma and gamma (as handshake actions). My first question is how do I determine what is a reachable state and what is not in a product of transition systems?

Secondly, how would I show that I used DFS on paper? I couldn't find any useful articles online about it and our lecture notes don't cover it for some reason.