How should a developer write unit tests for a private method in an Apex class?

What are the methods used to show input in classic and lightning ?

The sales management team at Universal Containers requires that the Lead Source field of the Lead record be populated when a Lead is converted.
What should be used to ensure that a user populates the Lead Source field prior to converting a Lead?

The following automations already exist on the Account object;
* A workflow rule that updates a field when a certain criteria is met
* A custom validation on a field
* A How that updates related contact records
A developer created a trigger on the Account object.
What should the developer consider while testing the trigger code?

A custom object Trainer_c has a lookup field to another custom object Gym___c.
Which SOQL query will get the record for the Viridian City gym and it's trainers?