It’s best to look and feel for simple signs of tampering. Look at the other payment terminals at the gas pumps for inconsistencies, as explained by the FTC.
If an overt attachment isn’t there, look for signs that someone broke into the terminal. These include the little lockable door on the pump not being closed.
Notify an attendant if you see anything unusual before using the terminal. Other ways to avoid skimming include paying with a credit card or using a contactless method.