I check my role before accepting the fp/ops, and if it's wrong, I decline. Simple.
It's very simple, people know what they are specced as and what equipment they have, especially the closer you get to max level...so check before you queue.
I for one am getting seriously fed up with joining groups where the healer/tank are in fact dps specced and either fail spectacularly to tank because they have low endurance and no mitigation or don't bother healing because they are too busy dps-ing. To say "oh but I get group faster if I queue as both" is a waste of everybodies time - groups containing these people have little to no chance of completing the content, so please just don't.
DON'T QUEUE FOR A ROLE YOU CAN'T/WON'T FULFILL!
If you have field respec and two sets of gear then that is a whole other story, fairplay to you for being flexible. IMHO, you should only be able to queue for multiple roles in FPs once you have field respec unlocked - this would help solve a LOT of these kind of mistakes/problems...