J Ren Rencarts:
It's good if I cannot visualize cause this is a good marker to look for to check to see if I am just in Excellence and fooling myself. I say the words 'coffee cup' in my head and if I see it then it is not PCE. I have to say that most of the time that this will just knock me out of PCE.
"Checking" to see if you can visualize in the way I think you're talking about is basically the same experience as visualizing (where else are you going to look besides "in the imagination", and how do you know where that is unless you're experiencing it?), so it sounds like you may be sabotaging yourself there.
Also, I experienced times when I couldn't seem to visualize anything, and yet it clearly wasn't a PCE. Early in my practice, I found there were times when not being able to visualize made me worried or anxious. Eventually that would mess things up and the visualizing ability would come back, but it was clear that there was a period where not-visualizing and negative stuff would co-exist. (It's quite possible that I had a subtle visualizing ability remaining that I was not able to discern.)