The automatic transmission fluid (ATF) dipstick on a 1997 Ford Explorer is located on the passenger side of the engine compartment. It's usually a long, slender dipstick with markings for "ADD" and "FULL". It might be near the back of the engine, often near or amongst other dipsticks and fluid reservoirs. Look for a dipstick with a handle that says "ATF" or "Automatic Transmission Fluid".
Important Note: The engine must be running and warmed up to operating temperature (check your owner's manual for the specifics on temperature) to get an accurate reading. Simply checking it with the engine off will not give you an accurate ATF level. Improperly checking or adding fluid can damage the transmission.