Check out Terms and Condtions (restricted countries list, refund policy, account verification process). You can ask all these question in live chat (if you don't want to search this information by yourself, or watch our reviews).
Look on the kinds of sports (is there your favourite), championships, types of odds.
Calculate bookmaker margincy for 1-3 matches (1/odds on W1 + 1/odds on W2 + 1/odds on draw). Average margin in Pre-match is 2-4%. If bookmaker has low margin, than its bonuses wouldn't be so good.
Check out bonuses and promotions. What are requirements to withdraw a bonus (qualification bet, wagering (X5...X10...), time). Terms and conditions to recieve bonus or promotion. Sometime it's a better idea to decline a bonus.