If you can get to it to measure cylinder bore, the 216 has 3 1/2" bore. 235 is 3 9/!6.
216 intake manifold has a "D" shaped cross section, while 235 intake is round circular tube.
216 right side pushrod cover goes all the way up to the rocker cover, while with most 235's the side cover stops at the head leaving head visible. Although there were some early 235s with the tall cover, so this wouldn't be a 100% reliable indicator.
Most 235's rocker cover is secured by 4 screws in the base. 216 rocker cover is secured by two studs in the top middle; with no screws in the base. Again, some early 235 covers secured with two center studs, so it's not a conclusive way to tell.
