As the binary options market is relatively new, options brokers pop up and some disappear quite quickly, so check how long they have been operating for. I personally use Binary.com who have been operating since 2000, are licensed and have won industry awards.
Are you trading in the UK? If so, look for brokers who have sterling trading accounts, a lot seem to be aimed at the US market. Another thing to look for is; do they have a UK telephone support number, so that you can speak to someone if you do encounter any problems.
Aside from trust, features differ widely in terms of support and training materials offered, deposit size, trade size, assets traded. Some also offer demo account so that you can try before you buy. So do your own homework first. Good luck