All the info (original configuration) is inside the optical cover like the others said. Also you can look up the serial number to find more info about it like the age and plant manufactured. But my eMac serial number says its a first gen Mac Pro.
because they changed the serial number at one point so you get that misleading info , so dont get the idea of selling your eMac as a Mac Pro now
but to the point part from that little sticker in the flap which is sometimes missing or if the flap was changed can be misleading there are no visible signs really apart from the first generations 700mhz had a silver ram door at the bottom , the later had a white one , but the last generations usb2.0 too had a white door, so unless you take them apart .. or power them up and ask "about this mac " they look the same really
and even the model number can be misleading even if its pointing out its a emac as for example the education only 1 ghz usb 2.0 model had the same MPN number as the 1.25 model usb 2.0 model