Look, you'll never really know (unless you do some proper sleuthing I guess).
I like to give people the benefit of the doubt. If you get treated like a liar even when you are telling the truth, there's no incentive to tell the truth.
Being suspicious and making false accusations could damage your relationship with her. My mother used to do this to me and it was around that time I started to seriously dislike the woman. And now I'm older and a mother, far from the "you'll understand one day when you are a mother", in fact, no, I judge her the shortcomings she should have overcome in the name of good parenting.
Next thing you know you are snooping around and reading her diary (I'm not saying you will but its the next step!!!)...
If you have serious concerns that she might have been keeping bad company last night, I'd focus my attention there, in a subtle way. Its fair to have asked her - but I would ask, and then give the benefit of the doubt, knowing that there is a chance that they are lying (as there is with everything) but that hopefully my parenting is up to scratch enough that there's no real need to lie.
And if there is, work on that.