How to use /oauth/userinfo api?

I was tring to use /oauth/userinfo api, but i didnt find the sample request about, and can i use this api by username and password? Actually im looking for a way to test username and password, if this api can`t be used by username & password, then is there a way to only test username & password? Or i can only use the token api to know if its correct?
Thanks in advance