These can be identified by the metal face plate and storage capacity of either 80gb, 120gb or 160gb
These can be identified by the plastic face plate, they are normally white or black but may be coloured as they are commonly modified.
These can be identified by the white plastic face plate and curved edges.
These can be identified by the one piece white face plate and 4 recessed buttons
These can be identified by the four petruding buttons surrounding the wheel. The first and second generation look identical only physical difference is the first generation has a scroll wheel that moves and the second generation has a touch wheel much closer to future generations.
These can be identified by the signature red click wheel and black faceplate. There are U2 editions of both the 4th and 5th generation iPods. (These will only be purchased as a U2 Edition if you can confirm the serial number as these are commonly faked).
These can be identified by the lack of iPod loop, the camera and the specific colours. 7th generation has the model number: A2178 and the 6th generation has the model number: A1574.
The 32gb and 64gb versions can be identified by the iPod loop circle on the back and the specific colours.
The 16gb can be identified by the grey colour and lack of camera.
These can be identified by the black or white bezels and less of a metal edge visible from the front.
These can be identified by the smaller metal edges compared to the first generation. The second generation has a model number of A1288 / A1319 and the third has a model number of A1318.
These can be identified by the larger grey metal bezel around the outside of the screen.
These can be identified by the coloured labels on the click wheel. These are available in 4gb and 6gb capacity.
These can be identified by the curved metal coloured case. These are only available in 4gb capacity.
To get more information, please check out the apple website linked below.