Those are considered illegal in US. CFD and like skirt the law here.
The big problem with bucket shops is that they tend to not honor the bets once you start winning too much as Jesse Livermore found out in late 1890s.
Still common today with all the fake forex exchanges. The rare winners have trouble getting money out.
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...
A bet between trader & brokerage that the price will move a particular direction, with no actual trading or stock ownership occurring
https://www.investopedia.com/articles/stocks/09/trade-a-cfd....
EDIT: It occurs to me this may not be quite what you meant after all, but I think the end result is fairly similar
Their biggest losers are probably making essentially random bets in such a stupid way that they lose all their money.
