How To Check if string is openssl rand -base64 32?

i need help, is there trick to validate string is using openssl rand -base64 32

here is some random output from openssl rand -base64 32

wbMcU+zrfwxOYJQlELE7hGWTd+0UFcIeGEnAKUdB2yE=

i searching like this, but i dont get the answer..

this post is similar like Check if string is UUID

but i don't know how to using if then fi to validate openssl rand -base64 32

im newbie, please help me.