Familiarising yourself with body language can help you get sense of whether a woman is checking you out. Communicating attraction non-verbally can be the first step in flirting, or it can be a fleeting glance. Even if a woman seems to be checking you out, looks can be misinterpreted, so proceed with caution if you wish to pursue a romantic interaction.
Notice if the woman in question seemed aware of your body. If you saw her looking at you below eye level, this is a good indication that she may have been checking you out. The length of the glance is important; if her eyes seemed to linger on you, this suggests that she was. Look at what you are wearing; if you are dressed in an unusual fashion, she may have been simply looking at your clothing.
Consider if there was an unusual amount of eye contact for the situation. If the woman is a stranger, did your eyes connect? An especially telling scenario is if she looks at your body and then immediately makes eye contact with you. If the woman is someone you are already interacting with, consider the length of time you held eyes. Think about whether her full attention seemed focused on you. Does she seem nervous after interacting with you? If so, she might be interested.
Notice whether the woman smiled at you. Although friendliness can be misconstrued as romantic interest, smiling in conjunction with prolonged eye contact can be flirtatious. If it is a woman you are talking to, think about whether she seemed to especially enjoy your jokes or stories. If she leans toward you in conversation, she may be interested in you. If she makes physical contact with you when talking, it may be a sign of interest.
Indications of romantic attraction can lead to dating. If you pursue further contact, be conscious that either party can lose interest at any time.
Verbal cues vary from person to person. Even if you feel confident that a woman is interested in you, there is always the possibility that you have misinterpreted her body language. Be respectful of other people's boundaries. She may be innocently flirting with no intention of pursuing anything further.