Count the number of splines on your input shaft. If it's got a 27 spline count it's a non-lockup transmissionm, if the spline count is 26, then it's a lockup style transmission, and the converters won't interchange.
If the converter that you've got now is a factory unit, it should have a decal on it.
The sticker could be a triangle shaped item, and it will say lo stall, if its round, it should say high stall on it. On a lockup converter, the sticker could be either round or sqare, and again will tell you if it's HS (for high stall) or LS (for low stall). The rpm difference is probably no more than 300-500 rpm between the factory high stall to the factory low stall. The converter will be an 11" diameter unit with a narrow ring gear, and was used in the leaning tower of power and the 318 engines.
If it's not a factory converter you've got, but a rebuilt unit, there usually is a Part number of some kind either painted on, or written on the hub side of the converter. If you send me a pm I might be able to give you further information on the converter you've got.
There are a number of high stall conveter rebuilders, some good, some not so good. Whatever brand you purchase, make sure that it is compatible with your engine/axle ratio/rear tire size set-up. As I've said in the past, a properly thought out high stall is a good $$$ investment, a poorly thought out high stall is a waste of $$$.
Good luck.