How would you be able to immediately tell whether a shale parent rock had been subjected to either intermediate-grade contact metamorphism or intermediate-grade mountain belt (regional) metamorphism?

On the basis of heat and pressure the rock undergoes transformation.

Explanation:

The contact metamorphism occurs when magma comes into contact with the already existing body of rocks. Regional metamorphism occurs in wider areas of earths crust. It occurs in areas where the rocks have undergone deformation at time of orogenetic events. Metamorphic grades refers to the changes in the processing of rocks form low to high grade.  The shale rocks are metamorphized under intense heat and pressure to form slate thereby increase in the grade quality. If the rocks are subjected to contact metamorphism then slate is formed and if its regional metamorphism then phyllite, then schist, and finally, gneiss are formed.