I’ve had a few times in my life where I’ve experienced true sexual tension without having any sort of discussion about it, and it was apparent to me after a bit of time that the feeling was mutual and not just in my head. As @mamalis said, when I felt it, I knew. That’s not to say I didn’t have some doubts as to whether I was imagining it, but it was palpable and impossible to ignore.
It was how I ended up falling in love with my first girlfriend (and vice versa). I was young and inexperienced, never had been in full-on love before, and at first I felt kind of guilty that I was having these feelings toward my good friend. I totally doubted my perception and feelings, but as time went on, it became more and more clear that I wasn’t alone in feeling this tension.
The only way to know for certain whether the other person feels the same is to ask them, or to feel them out in some way. I can only speak for myself, but the times when it was real, I was not imagining the tension between me and someone else, of course I doubted and questioned in the beginning, but as I said, with patience, over time, it became very apparent.
Everyone’s experiences may vary, though.. I think that I am an especially intuitive, empathetic person who is sensitive to various ‘energies’ in my environment and I tend to be really good at reading people. I have often allowed my imagination to get out of hand, to where I wonder, wrongly, if someone is more interested in me than they really are, but that is more a fantasy thing than correctly reading signs. When it comes to reading signs, or at the very least, something like sexual tension, my gut (intuition) guides me and I’m rarely wrong.