The first real cue, or "dream sign" as this website refers to it, that I taught myself was to ask someone in my dream a question. I would ask them a question that I knew for sure that I did not know the answer to.

For example, if I met someone whom I did not know their middle name, and asked them their middle name, I would either not get a response, the dream would alter course to distract me from the question, or there would be a major hesitation.

Any of these things I would notice as a hesitation, that my mind was trying to come up with an answer to a question, an answer that I did not know, and so either had to make something up or distract me.

So, I became aware of these things, these hesitations or avoiding of answering specific questions, as solid clues that, yep, I'm dreaming, and I can now exert control of my dream.