How to get "Layer Mask Hides Effects" from Photoshop Plug-in

I don't think there's enough information here to give an answer.  Is this a Plug-in that you're writing or intend to write?  If so, what type of Plug-in is it?

If you are writing your own plug-in, the header files included in the Photoshop SDK/API have a bunch of defines for the proper codes to use.

PITerminology.h has a large amount of keys that can be used to query for information.  PIUGetInfo is probably the function that you want to call to determine what you need to know.

If you're looking for information about a specific layer, you will want to make a new PIActionDescriptor, call sPSActionDescriptor->Make(*PIActionDescriptor), and then call GetLayerInfo on the layer that you're querying information about.

You can then call sPSActionDescriptor->Get<datatype> on the layer while passing in the key of the data that you want.