It sounds like you got the drum off since you're able to turn and count.
You can check for housing bend a couple of ways. First, use a straightedge along the axle tube, both along the top (er, bottom--trucks are spring-over) and along the back. If it lays flat as you move it side-to-side, you're probably good.
You can take it further by making marks on the axle tubes, say 5" and 24" from the housing end. Hold a straightedge between a marked spot atop the axle housing and another atop the backing plate (so your measurment location is consistent). Measure the distance from the tube to the straightedge at each of your marks, best done with a square resting on the axle tube. Do the same on the other side and compare measurements (you only need to compare the 5" to each other, and the 24" to each other). Doing the same thing with the back of the "cover" would tell you if you've got any fore/aft bending.
If the backing plates are visibly damaged or loose, you can do the same thing using the housing ends instead once the backing plates and axles are out. The measurements themselves don't really matter, just the consistency between them. There is going to be some variance for sure, but I'd bet you're OK. Even if the truck was dropped on the drum, most of the impact energy was absorbed by the suspension. If it did bend, it probably did it between the U-bolts and shock plates, which might not matter a'tall once it's narrowed.
