using an old rim
 

I was just about to build up a new wheel using an old rim and i was wondering what I should be looking out for regarding whether or not the rim is still good.

As a note this rim has ~30 000km on it has cracks around most of the spoke holes... so i assume its no good :(

Yes, less age and no cracks would be good starters.

Also little brake surface wear, and no sign of impact. Beyond that roll the rim on the floor and look for rise and fall at the top (egginess), then lay it on a known to be flat table (glass tables are great for this) and see if it lays flat or is curled like a potato chip.

I never re-use rims or spokes on my personal builds, and charge extra labor when folks ask me to reuse stuff. While a newbie might be tempted to save dough on his first builds, you're probably better off with a cheap but new rim.

I would reuse the hub and spokes, but not the rim. Since it has cracks now it can do nothing but get worse. Save your time and replace it.

yeah, don't reuse a cracked rim. I would think that was your reason for taking it apart! depending on the condition of the rim when you disassembled it, would determine whether or not you should reuse the spokes.

Place a value on your time as well as the components and you will see that often, the savings you realize by re-using questionable parts is quickly chewed up inn early replacement. For front wheels, this can be a safety issue as well. I would not re-use a rim unless it was almost new. You would be further ahead to find a bargain priced new rim.