The intention is to RC when something out of the ordinary happens, or when you experience a scenery change (like when you enter a building). And so on. Yes, dreams contain many things that you wouldn't see in real life but they still contain the same general elements/concepts: out-of-the-ordinary happenings and scenery changes.

This is an area that a dream journal helps with by allowing you to narrow down some of your common dream signs. Some people swear they don't have any but, I'd bet that every one has their own set of dream signs (and they tend to evolve over time). It may take some effort to realize them though. Then you'd want to ingrain in your mind the intention to RC (or simply recognise the dream) whenever you experience one of those dream signs.

Random RCs during the day is also a useful tool when done dutifully because it keeps the idea of doing RCs fresh in your mind and helps establish a habit of doing so. When you're in the habit of RCing every time you think about it and whenever something catches your attention then you'll be more likely to do so when it really matters just out of pure habit.