How can I tell if I lost my virginity?

Answer 1

There is no absolutely certain way. The classic definition of "virginity" was that an individual did not yet have sexual intercourse (male-female). For men, this cannot be determined in any practical way, and subjective, because one can have sex without achieving an orgasm (ejaculation), or conversely have an orgasm (ejaculation) without sexual intercourse, e.g. through masturbation.

For women, there is a greater chance of establishing virginity, physically, at least for vaginal sexual intercourse. Covering the entrance to the vagina is a thin tissue called the hymen ("cherry"), and it seldom remains intact following intercourse: it will be either perforated or torn, often causing superficial bleeding. So an intact hymen is an "almost certain" indication that penetration has not occurred. A perforated or torn hymen, however, cannot establish that vaginal intercourse has definitely taken place, as any of several incidents can cause this tissue to tear during post-pubescence, most typically masturbation or other external sexual activity. (Horseback riding was stereotypically suggested as a cause for damaged hymens.)

Answer 2

Virginity is a social and cultural concept with no biological marker to definitively determine its loss. It is commonly associated with the first experience of penetrative sex, but there is no universal definition. Ultimately, it is up to you to define what losing your virginity means to you.