The boat is a 1991 larson senza and I was wondering how i can tell if the engine is a 454 mag or just a 7.4L?

the 7.4 has oval port heads,the mag has rectangle port heads

Is there a way to tell well the engine is installed in the boat?

Serial number. Look on the back of the block, I believe on the right side.

Is there a certain digit or anything?

Metal tag on motor is ORANGE (rather than purple).

I think..............

It should be fairly well marked. That vintage, if it just says "7.4L Bravo I" on the spark arrestor, it's NOT a mag.

The intake manifold is raised higher towards the carb ,where a 330 oval port has a more flatter intake manifold,if parts (manifolds )has been swapped on engines its difficult to say ,but there is a way to see if the heads are retangular or oval port , on the end (front/rear side ) of each magnum/retangular cilinderhead there is a bump in the castingline ,all steel retangular port heads have these ,,bump,, where the ovals have a flat castingline ,see if you can find a magnum head ,and look for the bump you,ll see what I mean .

and to confuse things a bot more the 454/ALPHA available for a few years were called Magnums even on the airclaner but were just plain ole 330s not real Magnums.

There probably is a designated letter or number, or combination of such, that would determine whether it is a Mag or not. However I don't know what it is. If you give the number to a dealer they will be able to tell you. You may even be able to look it up on-line.

No.

The magnum engines have a high rise aluminum intake. The 7.4's have a steel low profile intake. If you pull a valve cover, the rect port heads say "Hi Perf" inside.