ssdave,
Pull out the breechblock. The inside of the frame of a forged action will show a smooth machined finish and be solid under the barrel shank. The cast frames will show a rough cast finish and are hollow under the barrel shank. Hope this helps.
Regards,
powderman

The cast vs forged frame question comes up over and over, with some folks like Dale53 saying they should be reserved for rimfire and leadball shooting a .32-40 IIRC in a cast action, and some folks even building big .40 cal+ BPCS rifles on them.  If I read (and understand ??? ) correctly in Dutcher, it appears that the cast actions were reserved by Marlin for what we would regard as "pistol class" cartridges up to perhaps .44 rf or maybe at the most .44-40 - these ALL in black powder loadings which were all that was commonly available during the Ballard's years of manufacture.  For reasons of safety (and of course, liability  ) I plan to limit my use of cast actions to .22 rf and perhaps .32 PISTOL rounds (yeah, I still want just one more .32 Mag!) and save the rifle class stuff for forged actions.  JMHO, but you can't go wrong being too safe!  8)

Froggie