There is a reason it's just hard to figure out sometimes. Start by keeping a journal of what was going on every time these crying jags start. Write down everything you can think of - where you are, what you're doing, who was there. Record any strange sensory things happening, a blinking light, lounge music, the smell of cooking fish - just try to make a record of everything you can think of in the environment. I bet if you do this you'll start to see a pattern emerging and you may be able head it off before it starts.
Hug him/her. Comfort him with your words and undivided attention. Try to relate and explain that you cry sometimes too. It will make him feel a little better.
His way of telling you something is frustrating him, with time you will learn the signs. Keep track of when it is happening, it will pay off and open a form of communication he can handle.
Great response of @A MyAutismTeam Member. In addition, I think its important that you start working to see what is comforting to your child. This is an opportunity for your child to see you as that person. It will expand your relationship if your child knows he can depend on you even if you don't understand what he is experiencing.
Haha! That should say "loud music" but maybe "lounge music" is just as appropriate :)