Look for a metal tag, often riveted or bolted to the side or top of the transmission case. It might be near the bellhousing (where the transmission connects to the engine) or on one of the transmission's side surfaces. The tag will have numbers and/or letters that identify the transmission model and possibly its serial number.
Because of the variability, a shop manual specific to your 1988 Ford Ranger's transmission is the best resource for precise location. You might also find helpful information in online forums dedicated to Ford Rangers.