Depends what kind of information you want to find… you can add a reference to the DLL in a project in Xamarin Studio and then expand the References folder & double-click on the DLL… this will open it in AssemblyBrowser and display the AssemblyInfo that was compiled into the DLL.
If you want to know what architecture the DLL was built for, run this in a Terminal window:
file insert_filename_here.dll
If it's a x86 (or "Any CPU") DLL, it will say:
insert_filename_here.dll: PE32 executable for MS Windows (DLL) (console) Intel 80386 32-bit Mono/.Net assembly
If it's an x64 DLL, it will say:
insert_filename_here.dll: PE32+ executable for MS Windows (DLL) (console) Mono/.Net assembly!