> Should I believe that there's no systematic discrimination? How would I be confident that there's not any?

Nobody can prove the absence of systemic discrimination. You also can't prove the absence of lizard people. The burden of proof rests on those making the positive claim. Look at how the technology industry tries very hard to attract women. Look at the outreach programs, the hackathons, and the scholarships from dozens of foundations and open source projects. Look at the mentorship programs available for women. That's some evidence _against_ systemic bias.

Now look at evidence _for_ systemic bias _other_ than the gender disparity itself. We're left with, what, rugs about meritocracy and a few off-color comments about dongles in a private conversation between two friends at a conference?

I just don't think the non-gender-disparity evidence is strong enough to support a claim of bias.

> should I discourage her from a field in technology because she likely has some otherwise hidden trait that will make her less useful or successful?

You should introduce her to many fields and let her decide which one interests her most.