Google and poker forums are your best friend when it comes to finding complaints about online poker sites. While there are always beginners who claim some poker site being rigged, there are legitimate threads on poker forums that can help you to steer away from a site or network that players have had problems with.
The first topic to search for is withdrawals on the specific poker site. Any complaint that seems to be a genuine one where the player has followed the T&C of the site should be a sign to proceed very cautiously. If that kind of case hasn’t been solved in a few weeks or months, by playing at the same site you’re basically risking ending up in that exact situation yourself.
If you find threads about a poker site or network sharing confidential information about their customers with 3rd parties you probably shouldn’t start playing there. Any information that you share with a poker room is confidential as long as there are strict license regulations in place with that operator. Even when you play rake races or other promotions, your whole name will never be displayed. Instead, they either show your poker alias or just surname and the first letter of your first name.
There are some exceptions to this though. For example on partypoker, there are now massive real-name tournaments where everybody can see who is playing at their table. These real-name poker events are increasing in popularity among tournament players.
Responsible gambling principles are applied on all licensed and regulated online poker networks and operators. If you ever find yourself playing too much or playing above your means, you can ask for self-exclusion or limiting the stakes that you can access.