Though prices have fallen a lot, there must be people that could only afford a dslr when bought second hand. This might be scaring but with a few tips you can have a clue if gear that is on sale on internet or at photoshops is light , moderate or heavily used by the previous owner.

The first part to look on a body are the buttons and knobs and jogdails, display('s) grip, doors :

buttons : Look at text near the buttons if it is a light used text will be easy to read , moderate used: text still is visible but on the most used buttons it s more vageu / absent. on heavily used gear it will/can be hard to read the text on the most used buttons.

knobs : feel how they act , look on the display if the desired element lights up e.g a menu knob should bring up the menu , look if you can see the menu clearly and if you can move through the menu. some knobs may have more than one function , most times like a WB knob you have to turn a wheel while keep on pushing the WB knob. look if the changes can be seen on a display.

on such actions you only notice action in the wheel it should only move from left to right and reverse but no action should be posible to move the wheel up or down.

One way to tell if there is a lot of use .. look at the grip, if the rubber feels weak and comes lose it is a severe or heavy used camera if not and the bubber looks and feels good there is lesser use to camera

The other way is look at the lens barrel, remove the lens or lens cap on the body and look to the silver ring , on normal and moderate used camera's it still looks shiny but none to some scratches may be seen. if the ring doesn't bling and shine and you can see a lot of scratches it is mostly heavy used

one advantage of buying in photoshops is that the camera is checked and some waranty is one it .

Doors: the should be opend easily and cloesd easily to if not than the camera will be intensive used ..

also look at connections for usb and cables and check the bottom of the body for readable serial number and eventually little buttons.

also take a the battery/ accu and insert it agian , this should be so that the battery doesn't fall out (with canon) at most nikons if you leave the battery door open the battery will fall out , this is not a sign of haevy use (i know a D50 and a D70 have that, may be more )

check for manuals , software , cables , memory card , always ask what is included , a charger should be included though you can buy a universal charger .

for lenses:

look how the lens looks , remove the caps, look through the lens , change aperture if there is a diafragma ring , look how the contacts look , they should look with out scratches .

mount the lens on the body , first zoom from small to wide mm , than look through the viewfinder, switch the body on, zoom manual look if you can see the change in the subject coming closer (when you zoom in > go to higher mm range) or the subject fading (eg zooming out , the image will be wider (less mm)

press the shutter half way test the camera , take a picture on Auto, P. TV(S), AV(A), M, and the scene modes , test the flash , review the images , look where you can see your histogram and how to zoom in on a picvture you took to see if it is sharp.

this guide is not complete but is imho a great way to check if used gear is good or not.
there is no guarantee from me , this guide is just to check , clevar guys and girls will always finde a way to decive , look before you react on several site's waht a regualr going /asking price is and keep that in mind .

I have bought most of my gear this way and these tips helped me though..