Check the Software Provider
If the online casino is not powered by one of the software providers, move on immediately:
Microgaming, Playtech, Cryptologic and, in limited instances, Real Time Gaming. The latter has had a bad track record with some dishonest operators, but there are a few that can be trusted, most notably Sci Fi Casino and Breakaway Casino
Check where the Casino is Licensed
Licensing is extremely important, If the online casino is not licensed by onee of the following jurisdictions, move on again:
UK and Isle of Man
Costa Rica (only in the case of Breakaway and Sci Fi Casino - avoid the rest licensed there)
Worse yet, if you can't find licensing information on the website, it's not even worth wasting your time.
Read their Terms and Conditions
If you like the look of the online casino, always read the terms and conditions and make sure you fully understand them. If they are not clear and simple to understand, don't even read a second time. They were probably written like that on purpose.
Terms and Conditions should be on the website, if not, don't waste more time.