Dana axles will almost always have the model number cast into the center housing someplace. Generally people go by looks or what comes stock.
I believe on the Dana 35 the ID is on the pinion end, driver side, bottom. There should be a 35 cast into the housing close to where the axle tube enters. I never really looked at a D30 for one but I'm sure it's there too.
I guess I'll work on an axle chart next.