How to test a void method in unity test runner with edit mode

I have a problem in testing my project since are most of the project are void methods that don't return anything.

for example, i have a register method which have some failure condition (when the password < 6, when the name has special character, when the email is in wrong format...etc) and the success condition will be directed to deferent screen and his data will be store in the database

in both 2 condition a success or fail message will be shown to the user.

do anyone has an idea to how test these condition ?