Are there two sets of bolts inside the rim. On a two piece rim, there are one set of bolts for the hub, and another set to hold the two halves of the rim together. I belive that a 78 ATC90 came with two piece rims origionally. The other way to tell is to check all the text on the side of the tires. Just about all tires say "Only for use on one piece (or two piece) rims" on the sidewall.
ATCnut
82 185s - sold
83 200x (hopped up)
84 200x (stock)
84 70 (lost this one to the DMV)
85 70 (soon to have lights, a foot brake, and CDI ignition)
84 200es (parts trike) - sold
85 350X (the best)
85 350X (siezed motor, dream of a poweroll kit someday)
85 250sx (frame was trash, got parted out)
86 250sx (the tractor)
86 250sx (a nicer tractor)
86 250sx (an even nicer tractor)
87 200x (parts trike, its really rough, a dog was chewing on it for a while)
87 250es Big Red (the tank!!)