George -
This site has some useful information about both formats. Also with a link on converting a Six-16 to work with 120 film. http://kodak.3106.net/index.php?p=212
The easiest and most foolproof way to tell them apart is of course to open them up and measure the spools and film gate.
They may even have the film size imprinted inside - one of those "Use Kodak 616 film" labels.
If you are trying to work from an internet listing, look carefully at the top/inside of the fold down front door, because there may be an indication of the model number there.
For many models of Kodak folding cameras, there are slight variations depending on the country of manufacture.
My Canadian manufactured Six-16 varies slightly from one manufactured elsewhere.
The film formats are very different. 616 is 2.5 inches wide, and the typical camera yielded a negative 2.5'' x 4.5". 620 is the same film as 120 - 2.25 inches wide. The length of the negative varied - 2.25 inches and 3.25 inches are two examples.
The 616 film is the same film as 116 film - the difference is in the spools.
are you talking about the cameras or the film? The Kodak 616 was a series of folding cameras that used 616 film and are clearly labeled. Brownie 620 cameras were mostly box camera and, yes, used 620 film. 616 film is about a quarter of an inch wider than 620. 616 is no longer available in any format, but you can modify the cameras, as noted, to use 120 or 620.