Do you see the striped pattern? These revealing measured alternating bands of light reflection will give you the answer. The smaller hole in the center of the film reel can also be a clue -- but careful, see below.
Now, look at this reel and note the light reflecting off the film's edge. The notable meaured line pattern is missing; and the center hole is much larger.
Did you figure out which is which?
The top image is Normal 8 film (a.k.a. "Double 8" or "Regular 8"), and the bottom image depicts Super 8 film.
The Normal 8 film has these measured alternating patterned, banding sections, of 25 feet in length, because after processing at the lab, the original film gets cut in half. The lab then wound it onto the reel and it showed up in this fashion. More detail - here
As for the reel's center hole size: Normal 8mm film has a smaller center hole than does Super 8mm film. Yet, there is something to keep in mind: Either type of film format can be mistakenly wound onto the wrong type of reel. That's why it's so handy to know how to discern the perforation hole size difference (or, that distinguished alternating, patterned banding of Normal 8 film, as described above.)