Turn off adblock and go to finance and forex oriented sites. You will see many ads for them at least in Europe.
A common marketing strategy for them is to offer heavy affiliate comissions and this leads to make-money-with-forex type of affiliate sites. That's another sign.
Bigger ones would be someone like Saxo Bank which started as a bucket shop in 90s and now is medium sized and somewhat legitimate now.
Another sign to be aware is the use of Metatrader software. Up to version 5 now.
I am sure there are turnkey solutions offering a whole bucket-shop as a service.
Even if there are honest bucket-shops you are still betting against the house not against other players in the market. Might as well bet on horses or sports then.
EDIT: nice discussion here on situation in UK: https://www.elitetrader.com/et/threads/is-there-an-actual-ma...
