Check the number on the buck tag bolted to the tailhousing.
It will have a number that goes 1352-52-XXX
The 3 digit number will tell you the year. Look it up here
http://www.therangerstation.com/tech_library/T5ID.htm
126 = 1985
141 = 1986
169 = 1986.5-1989
194 = 1989-1990
207 = 1991
208 = 1992-1993
Those are the most common when dealing with 5.0 Mustangs. There are other numbers, so check the table