I had a couple that came into my rented home as subtenants with my landlord's permission. They did not sign a lease but paid rent in the beginning. They left the residence after months of not paying rent and utilities, despite their promises to. I have documentation of checks they have written me before. What suffices as established residency? I also have their new address.

What is the best way to get my money back? Small claims court or getting a lawyer? What are my rights?

This is in the state of Georgia .

They were your tenants. With the same rights and obligations as any tenant. As long as you can show they established residency and paid rent you can prove the tenancy.

Small claims court would be best.

Next time, get a lease and reference checks.

Great. What suffices the best as evidence to show they established residency?

Proof of rent payment. Getting mail there? The landlord's statement that you asked for permission to sub let.

Okay, so is this unnecessary without a lawyer? Thanks for being so quick :)

Small claims courts don't require an attorney. Many don't even allow one.