By the part number which is in the parts manual and embossed on the radiator upper tank (left side facing the driver).
You can tell the condition on a new one by looking into the top tank and you can see the top of the flues. Not many, but some. Try filling it up with a garden hose. Hold your hand on the top outlet and let the bottom free flow. You shouldn't be able to fill it up.
On a used one, same thing. But look for obvious invasion of the flues by outside sources and internal corrosion. Use the garden hose again, and go same test as above. If it fills up then hold both hands on outlets and top it off. Remove the garden hose. Put your ear close to the top of the tank and at the same time observe the lower outlet (where your hand is). Sharply release your hand from botton outlet. Listen for sucking sound and observe the height of the out-going water. It should reach about 4 inches before it depleats and only just a little sucking sound coming from the flues.
Thats about the best you can do in the backyard. You can take it to the local radiator shop and have it boiled and rodded. Afterwards very tiny leaks at about 10 lbs pressure can be taken care of with a little powdered "Solder Seal." Or similar product.
These old radiators had some extra capacity and will run cool enough even if a few flues are stopped up or cut out, if too damaged (flues) to repair.
Good luck,
Charlie
