How do I find where a circuit starts in an older mobile home and how do I check the breaker to make sure that is working>

I have to suggest that an electrician be called in for this.

All circuits begin in the circuit breaker panelboard, and the only method to check any breaker is to open the panel and use a voltage tester.

There is deadly voltage exposed inside at many points, and you must be familiar with how to work in a live panel, have the proper tools, and proper protective gear.

Do you have the knowledge and tools to work on this panel?