Checking A dll's digital certificate.

I've seen Windows XP test their drivers to see if they were tested with Windows XP. I am assuming that Microsoft has code signed them with a certain certificate.
I want to provide a secure way of extending the functionality of a program I write now, with certain module's (OCX dll's) in the future.
I want to make sure in the main program, that whatever .dll is offered as an extention program, was written by me, and is in fact not some random any other file.
So I would code sign them with a specific self signed certificate of which I hold the private key.
Now what code would I need to put in the main program to check a file with a certain filename of having my digital certificate which I created prior and intend to re-use on each of my extention modules.
I'm sure windows holds the API's to use somewhere, someone will have written this already and I don't want to re-invent something, so where do I start looking ?
UPDATE: It doesn't really matter but all program parts are written in VB. So I'd prefer Visual Basic sample code, although with API's it doesn't really matter.