They Make No Mention Of The Future
If you've been seeing each other for some time and start to feel yourself getting emotionally attached, notice whether or not they ever make reference to a future together, whether it's mentioning a friend's wedding coming up in a few months or talking about potential spring/summer/holiday plans. If it doesn't, however, seem like they've been factoring you into their life, chances are they're not as serious about the situation as you are.