It's fairly common to switch parts around on those cars. I used to have a 1990 GT with plain LX tail lights. It's very easy to tell a GT from any other 1987-1993 Mustang because they have a longer body. If it's a long bodied hatchback, it's a GT. There was also a shorter LX hatchback and a regular LX with a standard trunk.
All of these Mustangs were available with the 5.0 HO V8, not just the GT.
EDIT: Wow, thanks for the pointless bad feedback. My answer is way better than the guy down below because I told you how to tell a GT from other types of 1987-1993 Mustangs and I explained the 3 different body styles that were offered in those years. I used to own one of these cars! Whatever, sorry for wasting time being helpful and correct.
GT's have longer bodies than LX and LX hatchbacks, period! They also have different bumpers and body kit. It's VERY easy to tell a GT from an LX.