There are very few hills where I live and I hardly ever shift, so the only time this happens to me on my Bianchi is when my husband has been working on it or riding it and has left it in some weirdo gear and I don't notice until we are on our ride. I can see the front chain ring okay (not that that helps, I think I have been off the middle chain ring once in each direction; the triple is totally wasted on me) but I can't see the rear, so if I really can't tell by feel I just ride ahead of him and make him tell me what gear I'm in.

If I don't start riding some place with hills I should probably sell that bike, because if all I ever use is "middle/middle," with occasional forays into "middle/left of middle" and "middle/right of middle," I could probably get by with a three-speed Raleigh.