There is quite a long incubation period for chickenpox so you wont know immediately. I have a funny feeling its 2-3 weeks but dont quote me on that! The child will only have been infectios before the scabs etc. formed, so if you were around him when he still appeared healthy, then you could have chickenpox. Im assuming youve not had chickenpox before, and I would say assume you could have caught it. Eat extra healthy for the next few weeks, lots of oranges for the vit C etc. just so you can give youre immune system an extra hand should you have caught it. And even if you dont get it, it wont have done any harm.
If you get chickenpox I think its about a week that you have to be off. Basically until all the scabs etc. have fallen off. Youre not really contagious while the scabs are there, but you wont want to be seen with them anyway!