How do you get tested for a yeast infection?
And can semen go through underwear?
The most common test for a vaginal yeast infection (assuming that is what you are referring to) is a swab sample of vaginal secretions on a microscope slide, mixed with potassium chloride, to look for hyphae (budding yeast forms) under the microscope.
Semen does soak through underwear, of course, but your question is whether underwear is porous enough to allow sperm to penetrate through to reach the vulva/vagina. The answer is no, sperm cannot penetrate through clothing, as long as there is that cloth barrier between it and your vagina.