How to decide if wiring in office / server room qualifies for replacement ?
Qualifies for replacement? You mean you have an old or inherited installation and you want to know if it's worth ripping out and replacing everything?
If the cables are fraying, splayed, jacket worn, if the blue cables are white (stretched) in places, if examining the cables entails wiping off rat or bird crap...chances are it needs to be replaced.
If the cable isn't at least cat5, it probably should be replaced.
If the network is running slow and it's not because you're maxing out your available bandwidth or it's a problem at the switches/routers, you should replace it.
If you have drops that aren't used or taped over because it's a quirky port or sometimes doesn't work or just plain doesn't work, you should re-run it.
If everything is running satisfactory with no "quirky" behavior, unexplained network drops, unexplainable speed quirks, I'd be inclined to leave it.
Get a tester and test every connection for proper speed and termination, using a quality tester. Beg or borrow one if you can't get one (although you should have one in your available tool kit.)
It also depends on the size of your installation. Are you talking about rewiring a building, or a small business? Are you a candidate for getting rid of your wiring or running just a few new lines and replacing others with wireless? Are you already planning on replacing your hubs with actual switches or managed switches, so that leaving ancient wiring in place would be kind of silly for the money you're pouring into the new equipment you're purchasing?
So there's a number of factors. If things are running well and there aren't any speed issues that's a reason to leave some things as they are, or re-run select cables. If you're seeing a lot of gremlins and your tester is showing a shaking head saying, "Uh-oh...sumthin' ain't right here..." that's a good indication you may need to start running wires. Make sure you do it properly with the correct plenum cabling or hire someone to come in and do it for you with cat6 or 5e, either of which should last awhile if properly done, and make sure everything is correctly labeled and tested. No corner cutting, no "I'll fix this later." Do it right so the next guy isn't having to ask the same questions you are, and make some maps of how things are run and laid out.
Well, there are types of cable for one... Cat3, Cat5, Cat5e, Cat6. To make the rating, it must be of a certain quality. Cat5e is typically good enough for most applications. There is also stranded, and solid copper as well.
Past that the only real way to tell is with one of those nice cable testers (not a continuity tester, a real one). Some NIC cards in some servers have the capability of testing a cable I believe, maybe Broadcom? Perhaps tell us a little bit more of what you're after here.