The size of a bulkhead is typically stamped on the plastic. However, aquariums rarely have bulkheads where they can be easily reached much less seen. If your tank already has something in the bulkhead such as a perforated pipe you can measure its outer diameter to make an educated guess on the bulkhead size (assuming there are no reducers used at the bulkhead).
Outer diameter PVC size:
- ¾ inch PVC is 1 1/16 inches (close to 1 inch).
- 1 inch PVC is 1 5/16 inches (close to 1 1/4 inches).
- 1 ¼ inch PVC is 1 5/8 inches.
- 1 ½ inch PVC is 1 7/8 inches.