Is there a way to tell these appart from the outside, without looking at the label (and not typing for sure)?
I don't know of any way of distinguishing them from buckling springs from just looking at them apart from the model numbers. AFAIK, any model M beginning with 7xG is a rubber dome, and all others that I'm aware of are buckling springs. So basically stay away from anything that begins with a '7' and you'll probably be safe.
I did get one of these for very cheap just for reference, but I don't have it with me at the moment so can't comment on how it feels compared to other rubber domes. I do though have a different IBM rubber dome board, not an M, which I'd consider better than the average rubber dome, not that it means much.
Regarding the comments on Topres, I recently acquired my first Topre board, a Realforce 86U, and it was one of the most disappointing purchases I ever made. Thankfully I bought used and at a hefty discount. It was my first time to type on Topres, and as much as I tried, I couldn't help but think, "they're nothing but rubber domes". I guess we all just have to accept that everyone has different tastes. For me though, regardless of how expensive it is, all rubber domes get a failing grade and are not something I want to use with any regularity.