Start with the licence, because everything else depends on it. Take the licensed company name from the footer, find it on the relevant public register, and confirm the brand you are on is listed as a trading name under that licence. If it is not, stop there.
Read the bonus terms before the bonus. The multiplier, whether it applies to bonus or bonus plus deposit, game weighting, the maximum bet while wagering and the expiry window are the five lines that decide whether an offer is worth anything.
Check the withdrawal rules next. Minimum withdrawal, any pending or reversal period, whether payouts must return to the deposit method, and what documents verification requires. Upload those documents at registration rather than at your first withdrawal.
Set your limits at the same time you set your password. Deposit and loss limits are far easier to configure calmly on day one than mid-session, and an operator that buries them is telling you something.
Finally, message support with a real question before depositing. The answer, and how long it takes, is the cheapest information you will get about how the site behaves when something goes wrong.