For flange style carbs....Stud / Bolt spacing is probably most important. If the bolt holes don't line up, the carb will need an adapter. Then consider the shape and size of the port on the cylinder head.
For spigot style carbs... (If there is some type of manifold / boot), then the inner diameter of the boot (or sleeve) will need to match the outer diameter of the carb.
Be less generic. The more specific you are, the more we can help. Tell us what engine or engines you are talking about. Show us pictures. Take measurements.