Key features of Mullins canoes are that they are heavily built, untapered ribs, the end grain of the stem is exposed between the inwales (most canoes cover it), the stem bands (if still present) are half round, as if someone cut a piece of copper tube in half, and the seat frames have extra pieces in the center space that act as clamps to hold the seat cane in place. I have also had Mullins canoes through the shop that were signed on the exterior of the hull and exposed when the canvas is off. The shape of the deck is also fairly unique to Mullins (see images below).
Dan