Well I'm in the minority here, but in my opinion bleaching/boiling/baking stuff is too much work unless your piece is really really small. Just find some fallen hardwood (basically anything but evergreen trees), give it a scrub, and then soak it until it sinks. If you need to put in your scape before it sinks, just tie it to a rock. Don't overthink it.
Rough tests for hardwood-ness:
1. If dry you shouldn't really be able to scar the wood (not bark) with a fingernail.
2. Cut off a tiny (less than 1" square) bit off the thick part of the piece and submerge it in a coffee cup with water. Microwave the cup until it boils then put it aside to cool and soak for a few hours. Repeat. If it sinks, your big piece will too eventually. If it doesn't sink, what you have is either rotten or not hardwood.