10. Good biochar has no smell. Even after it's been inoculated with something stinky, it will revert to no odour.
11. It will crush down easily into small fragments and fines. Incompletely charred pieces will remain intact.
12. If it's re-pyrolysed in a retort at 600C (average wood fire) the mass does not change.
13. If you burn it, the ash left behind is grey, not green or any other weird colour which would indicate metal contaminants like copper or lead.
14. The ash fraction left behind after burning is small, ideally less than 20% if you're using a woody feedstock. Biochar made from crop residues like corn stalks, straw or chaff will have higher ash content, and if you're using manure or sludge the carbon percentage can be well under 50.
Also, as a supplement to #7, good biochar can absorb 3x or more its own weight in water. Try it and see.
In general, chars with good adsorption capacities have the “feel” of good char: Properly carbonized wood forms a rigid, easily crushed material that lacks pockets of under-carbonized material. This material differs from the partially burned logs that linger after the campfire goes out. In addition, fully carbonized chars are also not particularly “greasy” to the touch. They are dirty and make copious amounts of black dust, but that dust will wash off one’s hands with just water. If it takes significant amounts of soap to remove the char powder from the pores of the skin, then the char has significant amounts of mobile matter, with the associated concerns discussed previously.. Other tests: Beyond these simple tests, it becomes difficult to accurately measure char properties outside a proper analytical lab. Attempting proximate and ultimate analyses without the proper analytical equipment is unlikely to yield any insightful results.
A large portion of my feedstock is white pine, despite crushing easily, it is still slightly resinous and will leave robust black stains on my hands. I haven't made it goal to get it where it doesn't, but I think if I did, I'd give up quite a bit of yield. So for now, I'll just keep using soap;)