You can write something on your hand like reality check - or something along the lines of that and eventually you will get used to it being on your hand, so then in your dream, when you see your hand it will say reality check and that will remind you to reality check.

Another way is if you are someone who fidgets is to get a ring or something and write reality check on it or just KNOW that it is for reality check and everytime you fidget with it, you will be thinking about your surroundings and asking if you are dreaming or not. You just have to make sure that you associate that object with checking reality.