to to distinguish if a d250 have been opened for repairs

how to? what are the signs

 

If the person opening the computer knew what they were doing and was careful then I don't believe it is possible to know they opened it.

 

even someone who knows what they're doing can cause some minor cosmetic damage. The keyboards for one aren't entirely easy to remove, I've done hundreds of them and still manage to leave a mark in about 1 in ten replacements, look for slight nicks in the plastic surrounding the keyboard and palm rest, also look for excessive "bounce" or movement in the keyboard, they're backed by a very thin piece of metal that can easily be bent, once taken out, they can be warped and may or may not go back in flat.

Examine the heads of all the screws carefully, if you see the slightest bit of shiny metal (they should be perfectly black) then chances are they've been opened. Also check the screws with a screwdriver, from factory most of them can be insanely tight, if they've been removed they won't be anywhere near as tight as from factory.

If it's a "used" machine, check for dirt and dust right aronund the edge of the screen. It's near impossible to clean the entire screen edge-to-edge without removing the bezel, if it's 100% clean then it's possible that it could have been removed to clean it.

Lastly, if it's got more than the stock amount of RAM, then yes, it's at least been opened to the point of being able to install a new RAM chip.