I am looking at a 1998 Checkmate the plastic shroud on top of the motor says its a 7.4 MPI. I am trying to figure what horsepower the motor is. Is there a way to tell if it is a mag or not by visual inspection

There was no "Mag" version of the 7.4 mpi. There were MPI version and TBI versions. It is 454 with cast internals and has 310 HP at the prop

Wasn't there a 330 HP version of the 7.4L? I thought I had one in my old 22' Scarab.

Yes the carb version said 330 HP.

7.4mpi= 310hp
7.4mpi/lx=330hp
454mag mpi 385hp

isn't it pretty much 330 at the crank and 310 at the prop for pretty much all the 454 non-mags
??