The intake manifolds look pretty different. The e motor intake runners are more narrow and I think there is a 7th fuel injector on top of the manifold which richens the fuel during cold wx startups. Find a pic of the intake manifold of either an e or an i motor. They look different enough that you should be able to tell immed.
The heads are different too. For example the e head has smaller intake ports, but it’s hard to see the difference with the intake manifold still bolted on. There is a part number ending in 885 on the i motor head below the intake manifold.
If you go to RealOEM.com you can probably find diagrams of the tops of the two engines that are good enough to distinguish the e intake from the i intake.
Oh, I forgot about electronics. The i motor has a redline of 6200 or so. The e motor is around 4000. In '88 they made a special es that had a redline around 5400 I think. What does your tach show?
Finally, check your DME (computer) under the passenger side of the dashboard. i motor DME’s part #'s end in 153 and 173, so if it’s an i motor you’ll find an i DME.