Long time lurker here, but I figured I would share what I look at before signing up anywhere. After a few bad experiences I stopped going by ads alone.
First thing is the licence: see which authority is listed in the footer, because some sites quietly block certain regions. After that I look at banking: card, e-wallets and bank transfer should all be there, and cashout times should be listed up front.
Never judge a site by the bonus alone. Anything above 40x rollover is a red flag for me, and some games count less towards it. A decent independent write-up with the current terms is worth a read here official site and it saved me some time.
On mobile I mostly use the browser version, and a good site loads fast and does not log you out every five minutes. Support matters too: slow or scripted answers usually mean trouble later.
Anyway, that is my routine. Only play with money you can afford to lose.