Candida in places that are easy to reach, like the genitals and the mouth, is very easy and reliable to test. It’s just like any other bacterium or fungus in those places: an easy to take sample (mucus / skin scraping) and a suitable growth medium is all it takes.
Any bacterium or fungus that lives in/on the genitals is easily transferred between partners during sexual activity. In that sense, they are just like any other STD. Symptoms may vary between man and woman and even from person to person, but transfer does take place.
If one partner really would have candida in the genital areas, treatment of just that one person is useless if the partner isn’t treated simultaneously. Mostly this occurs when the woman has a vaginal infection that consistently returns after treatment, just because her partner isn’t treated – even if the partner has no symptoms, he may be a carrier of the infection. So it really is important to treat both partners, even if only one of them has symptoms.