The quality of the frame and use
The first thing to look for when buying a new camera - whether there is evidence of its use. On the plastic parts and the lens should not have any scratches, scuffs and chips, the screws should not be traces of a screwdriver or deformed parts.
The next check is the number of captured still images. Even if the camera is in perfect condition, it might be time to shoot a few hundred frames and then give it back before the expiration of provisions of law of two weeks. Make a few frames and view the file names are numbers which are displayed indicate the sequence number of the captured images.
The following tests will check the focus SLR camera. If you buy a regular small camera with automatic modes, you can skip this step. In the photography shop staff provide special printout lined up with the marks in millimeters. Set the maximum aperture opening (they depend on your lens, it will be the small number, e.g., 1.6, 2.8, 4) and hover the mouse pointer over the center line, next to it is a proper title. After shooting to check that all the labels above and below the Central strips were read and eroded evenly. If such printouts, use a regular ruler and a white sheet of paper, making themselves desired mark as a line of focus.
To check for dead white pixels (these are sections of a matrix camera, which mechanically injured and did not participate in creating the image, so the display is always black or always white), close the camera lens cover, set the shutter speed to about 1/80 and ISO 100. Take a picture. It will turn completely black but a hundred percent increase in picture allows you to see all the defects of the matrix. To check for the black pixels you'll need to do the same thing, only on a white sheet of paper.
Battery
Take the camera in hand, will appreciate how comfortably it fits in your palm, and gently shake it - not loose in the battery (which means either poor fixation or substitution of the battery in some other, inappropriate size). By the way, the battery also needs to be checked - it must not be deformed or swollen.
With batteries for Nikon cameras there is one caveat. The manufacturer has officially recognized the existence of marriage in some of the series of batteries, leading to their explosion - this means that a battery at any time can cause irreparable damage to your camera. Please check the type and series battery. If the battery type is specified as EN-EL15, then the ninth place of the serial number can stand E or F, it will mean just defective batch. In this case, replace the battery.
