You might be able to look at the bottom and find the size stamped in it. Other than that, you could take an accurate measure and fill the carboy until full and see what it holds. That would take a while. If the price was right, I would buy them or maybe trade a few bottles of wine for them. Even if they were a bit bigger than 6 gallons, you could use them for a primary or for slightly large batches. If you make a scratch country wine, you could just make it a little larger. Then when you need to rack again, rack to a 6 gallon and maybe come out just right.
Also once you get your grape vines producing good, you could just make them full.
If the price was almost that of new, I would pass and get the sizes you really want.