Checking the vent?

Their comb will be a good indicator of whether they are getting ready to lay or not. The comb will get fleshy and red. 20 weeks is still a bit early depending on whether they are pedigree or not.
As for the vent, it looks a bit like a mouth and is quite obvious once you see it.  I'm not an expert and maybe someone will come along who will be able to explain things better.

How do i check the vent and where exactly is it?

The vent is a dual purpose 'bottom' Both the eggs and poop come from it, however the poop 'pipe' connects further down and closes off when the egg travels down the vent (hence no poopy on the eggs!).