Pellitised fungi

Laurie Fletcher spoke of this product in his 'beans is good for ya" presentation. beans is good
It got me cruizing the Net and I found this presentation which you may fine interesting
good fungi news
It deals with Arbuscular mycorrhiza development and function

I think the thing that I don't quite understand is how to be sure of viability between products. A lot of different outfits are selling mycorrhizal fungi but I don't know how to compare them well.  I understand that 'dry' processed fungi can have a long shelf life but how do I know that I am paying for lets say 80% viable fungi over lets say 20% viable fungi.

I think I lack the imagination of how it is manufactured and handled so I'm just trusting that when I buy fungi that I am getting good fungi and not already dead spores.