GM was commonly used in that year model truck but the mazda was also an option.
There were 3 GM engine models used: 4-153, 4-181 AND 3.0L engines and only 1 Mazda model engine M4-121G

one way to tell is to look at the valve cover

The GM valve cover is long straight and has filler cap near the rear of the cover and PCV valve at the other end of the cover.

The Mazda (M4-121G) engine is different in that it has a slant to the valve cover, a breather in the middle in the slanted part and the filler cap on the rear side of the valve cover.

Note: orientation may sound confusing but when i say "rear" of the engine it is the "actual" rear of the engine. It does sit in the truck backwards so keep this in mind.