How to simulate (run) the function

Hello;

I have an SQL function and I need to know how to simulate it (calling it and pass for it the argument and see what the
valueit returns), HOW? Can I do this from the pgAdminIII GUI or from the CLI?
 

This method help to check that the function is working fine specially if it will be called using the a specific
usernamewhich has access to that function.
 

Any advise?

Regards
Bilal