I recently shocked a new-to-knitting friend when I told her that not every yarn is designed to be washed by machine. If you don't know how to wash handwash socks, it's pretty simple:
1) Fill a sink or large bowl with cold water, add your favorite washing stuff (mine is soak), add socks.
2) Go away for 20 minutes or however long it says on the bottle of your favorite washing stuff.
3) Rinse (if your washing stuff needs to be rinsed).
4) Gently squeeze (don't wring) water from socks, wrap in a clean towel.
5) Stomp on towel, pretending you're stomping grapes in the French countryside (is that just me?)
6) Hang socks to dry the rest of the way.
About half of my socks are handwash and the other half are machine-washable. Tip: Even if a sock yarn SAYS it's machine-washable, you'll get the longest life out of your socks if you hand wash them all, every time.
Most of my handwash-only socks are distinctive colors or styles, but when I have a rather plain black sock that needs to be handwashed, I build in a little clue to myself. Do you see that tiny line of red on the otherwise black sock in the lower-right of the photo? That's my reminder - handwash these socks. I simply inserted a single round of red near the toe of otherwise all-black socks. It stands out when I'm sorting laundry, but is invisible when the sock is inside a shoe.