Spotting married men:
Try and phone them at home on the evening on a landline.
Ask them what they had for dinner - just out of interest, not an interrogation. Single men are less likely to cook complex dinners and will eat ready meals or fast food.
Men on free sites are more likely to be married than paid-for ones, because they're worried about credit-card bills being found by their wives.
If they say they don't have a profile photo because they're embarrassed about colleagues seeing them online - I'd doubt that. The only reason to be embarrassed about online dating these days is if you're cheating.
If they say they're separated - especially if they're 'sleeping in the spare room' then they're likely to still be married. Some men are, of course, separated and going through a divorce. But that's both an amber flag about being married and they're probably not in a fit state for a relationship anyway.
I"m posting all these reasons to be helpful to you lot, of course, but also for selfish reasons. I'm one of the decent single blokes on the OD sites so I'm just hobbling my competition a bit. :)
(Although I'm neither single nor on OD sites at the moment. 4th date with the lovelywoman next week, and lots of messaging and skyping while I'm away on business, and I haven't checked my Match inbox since I met her).