Usually by having their RNG audited and/or open-sourced/testible (though that has its own potential problems) followed by marketing and support and time in existence.
The newer a poker site is, the more likely it is to be shady, simply because it hasn't had anecdotal reports of it not being shady from actual players.
The newer a poker site is, the more likely it is to be shady, simply because it hasn't had anecdotal reports of it not being shady from actual players.