What should be checked on the inside sidewall of the tires?

Checking the inside sidewall of the tires for tread separation is crucial because tread separation can compromise the structural integrity of the tire, leading to potential blowouts while driving. It is essential that the tread adheres firmly to the tire; any signs of separation can indicate that the tire is in poor condition and may fail during operation.

The other options involve severe issues that would typically be more visible on the tire's surface or outside. For instance, cuts or bulges, visible damage, and leaks are critical factors in assessing tire safety, yet they do not focus specifically on the condition of the tread itself, which is the key aspect indicated in the correct answer. Paying close attention to the tread condition ensures safe vehicle operation and helps prevent accidents caused by tire failure.