If I see a pellet gun in a pawn shop or gun shop, I ask if I can see how it shoots. You can get a rough idea of the power by shooting a pellet into a thick paperback novel. I use those romance novels that my mother-in-law likes to read. Those are denser and should stop pellets better.
It's not a chronograph, but will tell whether or not the gun has any power or if it sounds like it's falling apart. Just get the ok from the shop keeper and explain that dry firing will ruin the gun, unlike a .22 rimfire.
That vintage of Diana's usually have leather seals. Sometimes those can dry out and lose compression and shoot a bit weak. But a little silicone oil or home made concoction (search on this) can restore it.
See what kind of scope it has. If not an airgun rated scope or one of those thin 20mm tubes, talk them down as it's probably nearing the breaking point or, in the case of the skinny ones, next to useless.
The 45's were Diana's answer to the first wave of mangumitis. While called a magnum it's probably in the 700's. Not like the "1000 fps" guns touted today. Still a nice gun, though.