FINDING THE PROBLEM

I'm not sure what you mean by "Diagnostic Tester". It is worth it to have a mechanic perform a diagnostic test of the charging and starting system if the vehicle is having a hard time starting. There are many things that can seem like a bad starter, so it's important to diagnose the problem to determine the cause, otherwise you may replace a part that doesn't need to be replaced.

take it to advance auto part store and get a free diagnostic

you dont need to check diagnostics for a starter, most battery checkers can perform that task, if the battery is ok & the car cranks slow check the voltage at the battery while cranking, you may also just have a bad connection or cable