How do experienced cyclists know which gear they are in without an optical display?
All the elite riders I have encountered just know. I think its part of the attention to detail necessary at that level. You almost never see them looking down to check (that would be showing a weakness and giving oponents an opportunity). 

But if you mean can the rider say which cog they are using at the front and rear? then the answer is that often we can't say. 

Usually we know by memory which front cog we are using. And as I change the rear gear, I know by the different feel of the ratio whether I'm near the top, the middle, or the bottom of the cluster. I very rarely try to change gears past the last one. 

The bike knows. That's enough.