Just look at it. If there are three of the select pins to either side of the cartridge slot, you have a six pin. If it has two on either side, it's a four pin. Nothing technical or hidden about it. Some people just don't realize that there are two versions of the old 2600 (check that, three versions, see below).

The extra two pins, as I recall, are the difficulty setting switches for each player. On the four pin model, the pins are actually just black thumb switches in back by where you plug in the joystick.

Here you go...

Six-pin 2600

Four-pin 2600

Then again, there is also the 2600 Jr.