I bought a bucket on craigslist that was for fermentation. When I got it, it looks like a mess and has no labels on it. I want to be 100% it is safe to use for fermentation. How can I tell? Is there a particular plastic or symbol I am supposed to look for?
If the bucket is a mess you should be cautious about using it to ferment in - the main downside to buckets is that easy access to the internal surfaces makes it easy to scratch. Scratches are potentially difficult to clean, and so are a possible source of contamination.
Here is a an article on identifying food grade buckets. tl;dr - look for the recycling symbol on the base - 2 is best, then 1,4 and 5.
In my opinion, buckets are close to the cheapest part of brewing and you should not take chances. a reputable home brew supply house will sell them for around $15 with a lid and spigot. i would hate to see you spend a day brewing, a day of bottling activity, 2-3 weeks fermenting and a couple weeks in the bottles and find out that you have 2 cases of gross beer to save $6-$8.
I would only use craigslist for things that never go near my mouth :)