Automatic transmissions require the use of a specific type of transmission fluid recommended by the vehicle manufacturer. Failure to utilize this specific type of fluid can result in a number of problems with your transmission. Becoming familiar with these problems can let you know when your mechanic has made an error.
Using the wrong type of transmission fluid can result in the fluid becoming "burned" too quickly, resulting in visual changes to the structure of the fluid itself. Check this by looking at the dipstick for your automatic transmission fluid. If the fluid has a sickly and discolored brown appearance, the fluid is going bad.
Using the wrong type of transmission fluid can result in the fluid developing a bad odor at the same time, allowing you to recognize the condition before it is too late. If upon checking the dipstick the fluid smells a bit like burned toast, the fluid is bad.
Leaving the wrong type of automatic transmission fluid in your vehicle can result in your transmission receiving improper levels of lubrication, resulting in the engine components for your transmission sustaining excess wear through friction. This can completely destroy your transmission, so exercise due caution.