Ok, I figured out that you can get that information via gacutil /l

You need to build your editor(s) and designer(s) into an assembly (dll)
through a .NET 2.0 class library project (a full framework project, not a
compact framework project). You then compile this full framework class
library assembly ensuring that it's strong named using a generated key (this
can be easily done in VS 2005 through the project properties). This assembly
will need to be installed into the GAC (gacutil /i [assembly name]). Once
the assembly is installed into the GAC you can view all the information that
you need to indicate the "Editor" tag within the xmta file. The information
is accessible within the GAC by right-clicking on the assembly and viewing
its properties.

Note that when creating a custom designer for a smart device custom control
you should inherit from the
Microsoft.CompactFramework.Design.DeviceControlDesigner type which is
located in the Microsoft.CompactFramework.Design.dll assembly.