Ask for a damaged-parts report and a breakdown of the work that was performed. If you want to inspect the damaged parts pulled from the transmission, be sure to ask the mechanic when you authorize the work, not after. As with any contract, make sure you understand all aspects of the work. If you give your authorization over the phone, understand exactly what work you are approving. Phone authorization is a binding commitment.
Any warranty is only as good as the company that stands behind it.
Your transmission repair should come with a standard written guarantee backing the work. Some industry organizations offer nationwide warranties. These warranties offer assistance throughout the country from participating member locations and well-established service centers. Some warranty programs may be administered by outside warranty and insurance companies.