How do I tell if my 340 is internally balanced?

Crank looks cast because of the thin parting line. OEM comes with a balancer that has a raised counterweight cast into the front of it that say "FOR 340 CAST CRANK ONLY", or something like that. Converter should have weights welded on. Since it has TRW pistons, it was obviously rebuilt. Mallory metal slugs are usually installed in the end counterweights to "internally balance" an originally external balanced assembly. If they don't have any slugs added and the TRW pistons are a good bit lighter than OEM, someone took it for granted that they were light enough & assembled it with a neutral, forged crank type of balancer, or it was checked on a machine and they lucked out with the TRWs being lighter enough so no additional machining was needed anywhere for use with a neutral balancer (and converter).

The existing holes in the counterweights are normal factory fine tuning of the balance. Factory balance wasn't as accurate as a decent engine shop, but it was "good enough". Rods don't seem to be touched up by a machine shop either.

Do you know if there was any vibration when the engine was run? Or was this a "bought" engine? But to make sure it's "right", a shop with balancing capabilities would have to weigh everything & spin the crank.