Wow,looks like we are going to have a deluge of "Look at what I found!" threads!! Awesome!
Alrighty, now for your questions.
Knowing the wood is safe has a lot to do with knowing where you got it from. If you are collecting right downstream of a waste treatment plant, you could be getting some nasty stuff in your wood. Most of the nasties can be taken care of with a good boiling, but chemicals and such tend to last not matter what you do. You can find out the quality of the water in an area a couple different ways. The easy way is to shoot an email to your state water services, like the fish and game, and ask them about the area you are looking to collect from. If you want to spend more time and find it out yourself, you can sample the benthic macroinvertebates and calculate the EPT ratio, or do a family level study.
Something like this could help you do a study of your own.However, it is not likely to be as accurate as the long term data the state will have.
a lot of times, you can use common sense to get a general idea of water quality. Is the water cloudy or smelly? Are there dead fish on the banks? Is there garbage and scum in the water? Are the only insects in the water worms, midges and snails? All that would be a bad sign.
To make sure the wood is not just going to disentagrate, you need to find really HARD wood. I am often dissapointed to find an awesome looking piece that crumbles in my hand. If you can easily jab a fingernail into the wood, it is too soft. You want it as hard as possible, so it will last a long time. Soft wood isn't inherently BAD, but it will break down very quickly nd make a hugemess in your tank. If the wood is mostly hard, with just a few soft spots, you can cut off or gouge out the soft areas.
Once you get your wood, it will still need to be cleaned really well. The first thing I do is take a garden hose to it. As Rbishop mentioned, a power sprayer would be ideal. Get a rough brush as scrub scrub scrub. Then, I let it dry COMPLETELY. A large, water logged piece is going to take a while, but leaving it in the sun will speed up the process. Then, if possible, you want to boil it. However, with large pieces, this may not be feasable. In that case, a good, long soak is advisable. This removes tannins, and waterlogs the wood so it will sink.
After that, you are good to go!
