It truly depends on what you mean by "good" and "bad."
You can do everything right in the pyrolysis stage, but if your inputs are coming from clear cut forest, would that change your opinion as to what is "good" or "bad"?
The process itself is what separates biochar from charcoal, as well as biochar from activated carbon.
Many factors come into play in terms of the final biochar product. One of the best resources for understanding biochar, IMO, is the Biochar Journal
. They do a very good job explaining what biochar is, what biochar isn't, and how it can be used. Like the permaculture adage "animals above plants," biochar needs to be inoculated with nutrients, water, and microbes before being applied to the soil. This can be done in many ways, such as including it in compost, vermicompost, adding feed grade biochar to livestock's diet, mixing with urine, adding it into the bedding of livestock's quarters, etc.
One of the founding entities of that journal has also played a huge role in getting the European Biochar Certificate (EBC)
started. If you go to the EBC's website, you can find their requirements for certified biochar. There are only a handful of operators who have willingly undergone the process so far, but I think it will grow. Their criteria are strict, but clear and reasonable.
I have asked some biochar producers to provide me with information on the nutrients, heavy metals, pH, and other factors of their products. Some simply do not respond after you request the information. I think that having a standard- whether it be the EBC or something else- is very useful for consumers. All biochar is not created equal and, like the mycorrhizal fungi industry, there are too many people wanting to make a quick buck passing off inferior, diluted, or otherwise compromised goods.
At the end of the day: good biochar, IMO, is made from sustainable sources, in energy efficient pyrolysis systems. It is checked for contamination by heavy metals or concentrations of other potential health hazards (especially if it is to be sold). The biochar has a high surface area (say, at least 150m2/g) and has been put to at least one use before going into the soil.
Bad biochar is easier to make than good biochar. There are many deal breakers for me. Say the input source is in no way shape or form sustainable, then I'd say it is bad. If the inputs are transported over very long distances, again, it crosses a red line. If the biochar is being made on an industrial scale, say more than 2 tons a day, and there are no environmental safe guards, bad. If inputs are mixed and records for each batch are not taken. If the company producing it doesn't check for lead, cadmium, etc. or, if it does and attempts to cheat, then it is not any good.
So, as you can see, there are many ways of looking at biochar production and everyone's standards will differ.