If a woman is trying to seduce you, she probably won't tell you what she's up to. However, she will likely give a number of different signs that might include body language, phrases with double meanings and flirting in general. In order to tell if a woman is trying to seduce you, you need to keep an eye out for these signs, because if you don't act on them the woman will likely assume you are not interested and move on to someone else.
Lean in to her to see how she responds. If she is trying to seduce you, she will already be leaning in to you, so your leaning will result in her leaning even further.
Listen to see if she is laughing at your jokes. If she is being seductive, she will be having a good time. Laughing and smiling are signs that she is enjoying herself, which in turn may be signs that she's attracted to you.
Look for open body language --- arms open, uncrossed legs, and generally leaving herself physically exposed are all signals that she expects you to notice.
Look to see where she's looking. If she's out to seduce you, her eyes will almost exclusively be on you. If she wants to just be friends, she will be looking all over hte room.
Listen to see what she talks about. Does she ask you questions? Does she seem interested in what you do for a living and what your hobbies are? If so, then she is interested in you, or at least showing interest in what could be an effort to seduce you.