Is my cassette worn ? if so what can I use to replace it ?
 

Alright long story short, I left my bike at home after last summer and left it to a friend to ride and keep maintained. When I was riding it I only experienced the "skipping" once, I tuned my derailleur then everything was fine. My friend called me today as my bike will be shipped out here next week to give me an update and said over the last month a few of the gears have been skipping really badly, he got it tuned up and put a used chain on it because the last chain broke. Is it possible that it is the chain being the wrong size or are the cogs worn out or anything else ??? I got him to message me some photo's so that you guys could take a look. If it is too worn out which it looks like is a very real possibility what are my options for replacement. I have c record hubs and the current cassette is 12-21 Im not sure what family it is in but it is campy and came with the hubs when I purchased them. It's and 8 speed so can I use any old campy 8 speed or are there things I have to watch out for, I just really want to have a cassette for when the bike arrives just in case and want to know what to get, but don't want to blow 100+ bucks on a cassette that won't work with the bike. ????
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You can't determine cassette wear by appearance.
You know it's worn out if you put a new chain on and it skips.
Any Campy 8s cassette should work.

I have been told that, just to be on the safe side I want to purchase a cassette, so if I have a campy 13-21 can I use any 8 speed campy on the c-record hub or does it have to be exa drive ?

To my eye, the cogs which can be seen, look to have little or no wear. The chain, however appears to be too "thick" thus I wonder if it is the correct size and is maybe meant for a 5-6 speed rear. Try a new nine speed SRAM, KMC, or Z (or other brand) chain first. I really think the bike has the wrong chain.

Your friend put a USED chain on the bike? That will probably the cause, use a new (8 speed) chain and test the bike.
You can use any new 8 speed Campagnolo cassettes, they are still available, here in Holland they are about 35 euros.

Yeah. Ideally, chains and cassettes/freewheels should age and be replaced together. The trick to keeping an old freewheel/cassette going for as long as possible is to replace the chain with a new one fairly frequently so the wear is minimized. Installing some random used chain from another bike is a really bad idea.

I'd vote for the chain being the root cause. (Well that and having left the bike behind ;) )

yeah I wasn't too happy when I found any of this out, I wish he would have called me when the chain broke and I wouldhave had it send out here then!

I have one more question, I am rocking a 13-21 cassette right now , can I just buy the cogs ? Im looking at 12-21 cogs would it work ? If I have an 8 speed cassette can I just put any 8 speed set on it or do I have to buy the full cassette with the splines ?

You have a buy a complete cassette. The cog wheels are not available separate.