It is not possible for you to know whether you had precum or not but most important thing is that you did not have intercourse.Now when she has already had her period a after sex that means she is not pregnant so there is nothing to worry about.Technically you need to actually have intercourse(penetration ) to make her pregnant but in your case this did not happen and now she is in her new cycle which started with her period.
A girl can actually still have her period if you had sex days before! Providing she was ovulating which is possible she could get pregnant. It takes 7-10 days for implantation to occur. Get your girlfriend to do a test in two weeks just to put your mind at ease.. The fact u didn't have full on intercourse can make this more unlikely but still possible. Good luck!
That's not a likely scenario for getting pregnant -- but it does happen occasionally. I suspect you're in the clear but it really depends on when she was ovulating. Did she get her period on the day it was normally due?
Emergency contraception is one option to consider but it's hard on your body. I don't recommend taking it unless she's sure there's a high risk of pregnancy (like she felt ovulation or noticed a lot of cervical mucus around the time you were intimate). If she got her period on schedule, she's most likely fine. She can take a pregnancy test in a couple weeks for peace of mind if she wants.
In the future, keep in mind that any sexual activity that involves your parts touching, inside or outside, comes with the potential for pregnancy and STDs. You can do your part to stay safe by using protection, every time. :-)