Sometimes you can tell sometimes you can't. I've had the yellow lines in the middle of the street goto a complete zigzag pattern----that seemed real to me I thought that I had entered a parallel spirit world(the delusions make the hallucinations more real). I had three girls walking around in metallic kimonos switch to jeans and T-shirts after I glanced away(again blamed this on a transition to the spirit world). Those happened while I was really sick and not on meds yet.

On the meds I can tell----the meds seem to stop the delusions pretty well even if they dont get 100% of the hallucinations. So an example of one I could tell was a piece of computer circuit board designed into a pendant necklace. The necklace was floating in front of my face in perfect focus and in the light even though it was night and therefore dark and I had my glasses off so I should not have been able to see it clearly.

So basically if you're sick enough you can't tell because logic doesn't work, but if you don't have the delusions you should probably be able to tell because things are off somehow.