Pay attention to how the girl behaves in communication. There is a perception that virgin is much more modest. In conversation with a man they often blush, not always immediately find a common language, provide the initiative guys. And not always it is a clear sign: some girls specially behave differently to please his companion.
Try to start a conversation about previous relationship with a girl. Ask if she had men up to you how much time she met, etc. Usually virgin try to avoid such conversations and answer questions vaguely. Sometimes they even write love stories, but if you want them you can find a lot of far-fetched and naive.
Look at how the girl behaves, if you try to touch her, hug or kiss. Inexperienced and innocent girl behaves in such moments it is very awkward, and this suggests that previously she did not admit men to myself too close.
Invite the girl to spend the evening at your or her house (if you've been Dating for a while) and see her reaction. If she is a virgin, it may be scared of this proposal. If she still agreed to try to move on to more intimate caresses – while kissing, touch her Breasts, running his hands over her body, etc. Even if the girl behaved quite normally, she may begin to exhibit noticeable awkwardness and even specifically to stop you. This again may indicate her innocence.
Contact your friends or girls, if you know them well, and ask them about if she had someone to you or not. Usually this information is not hidden, and if you are not a good first impression, girl friends will tell her the truth, to help your relationship.
Invite the girl directly to have sex, if you've been Dating for quite a long time. Tell me that you have feelings for her and want to become even closer. If you like her, and the moment is appropriate, it is unlikely she will say no. However, if the girl was no one she could admit it to herself, since is experiencing a strong emotion in front of such an important moment.