Straight back?

I feel like this is a stupid question, lol. But I'm unable to workout in front of a mirror so how can I tell if I'm keeping my back straight? I have back problems and tend to slouch so when someone says "keep tour back straight" I dont know what that feels like. I dont know if I'm bending too far back. I dont know if I should tuck my tail bone in or out.

Is there a way I can feel if my back is straight?