How can you know if vaginal discharge is just a yeast infection or something else?

See your GYN doc: Yeast is a common vaginal infection. The other two are bacterial vaginosis and trichomoniasis. The last one is sexually transmitted, the first two are not. Yeast much more often than the others is characterized by itching and often also a white to pale green discharge, which is occasionally abundant, although sometimes not so much. One can try over the counter creams which are often effective.