How do I tell if a woman is menstruating? I've tried asking, but they usually take offense.?

I know that I am allowed no contact with a woman while she is in her period of menstrual uncleanliness - Lev.15:19- 24. However, I'm having a difficult go of it. What does everybody else do? 19 ‘When a woman has a discharge, if her discharge in her body is blood, she shall continue in her menstrual impurity for seven days; and whoever touches her shall be unclean until evening. 20 ‘Everything also on which she lies during her menstrual impurity shall be unclean, and everything on which she sits shall be unclean. 21 ‘Anyone who touches her bed shall wash his clothes and bathe in water and be unclean until evening. 22 ‘Whoever touches any thing on which she sits shall wash his clothes and bathe in water and be unclean until evening. 23 ‘Whether it be on the bed or on the thing on which she is sitting, when he touches it, he shall be unclean until evening. 24 ‘If a man actually lies with her so that he