Quite often a new sportsbook is a new skin offering the same odds as another provider. That allows a company to reach a new audience whilst maintaining the same back end and admin staff thus keeping the costs and the risks as they were.
To keep up to date with new sportsbooks opening there are two things you need to do.
1. Find a site that reviews new sportsbooks
2. Set up a google alert for key search terms, like 'new sportsbook'
Using a bookmaker review site is a great way of taking some of the risk out of signing up at a new book. If you deposit at William Hill, the chances are good that you'll get your winnings back promptly and with no issue. Signing up with an unknown quantity can sometimes be a leap of faith. To mitigate the risk you can let someone have a go first, and then stand on the shoulders of giants and follow in their footsteps (if that's not mangling too many analogies!).
But if you really want to be on the front line then do some research yourself by setting up google alerts. You won't always hit all the new sportsbooks, but you'll often be lead to some interesting new information if nothing else.