from what i've experienced , here are some tips to recognize them, but it's just an advice
a high grade board looks like a military grade board i.e many components (as IC , transistor ,etc...) have a great amount of precious metal in it that can be refined for a profitable return in a refining point of view
a mid grade board is a electronic boards from small electronic devices. These boards have very few small chips and usually contains big portion of passive electronic components.
Characteristics:
Boards from the telecommunication industry
Few IC's on the board
No visible gold plating
Can carry small metal pieces
Large portion of passive components ( e. transistors, batteries)
a low grade board contains a few PM and lots of BM
this is a general view but as you experienced it you will see
of course all of that is comparative and i'm not an expert so for sure others can tell you better advices
cheers.