Correct, sexual tension is a two way current. One person horning up another without reciprocation is not sexual tension. It's lust. Or torture, depending on your mood and how soon after you're able to masturbate in a public restroom.

Like most wonderful things in life, sexual tension is, for the most part, intangible. It is a feeling. It is a millions specks of energy that fly between two enamored hot and bothered souls. That said, some tip-offs might be:

Both parties often gaze into each other's eyes.

Both parties try to look good all the time.

Both parties notice they're being stared at.

A back and forth display of kindness. Example: Willingness to share last bites of ice cream or the best blueberries in the bowl.

A mutual effort to extend moments, conversations, hang-outs. Example: Let's each smoke one more cigarette before you go, OK?

A consistent drive from both parties to touch "platonically". Examples: "Let me help you with that." "Ooh, your hair is so soft. What do you use?"

And lastly, either consistent talking directly about sex or talking around it: sexual innuendo, flirtation and off the cuff remarks. Examples: I haven't been laid in months. I'm gonna explode. OR I think Scarlett Johansson has such a great ass, but her boobs get all the media attention, right?