The vagina is a beautiful and magical thing. You cannot tell if the women's vagina has been in intercourse. It will look the same unless the person has just then engaged in sex. But if that is the case the women would also have flushed cheeks and maybe messy hair. The vagina might be red or swollen but you can't really go by that because it could look that way if the weather changes or she just itched it. So, sorry there is not a way to know. Just find someone to trust and tell you the truth.
Can you tell just by looking at the outside of a young woman's vagina if she has had sex or not? No.
Now that might not be so true when looking at an older female who has had a lot of um, sexual relations...