Listing properties added with add-member

Add-member is a great way to extend object properties in powershell.  How can I determine what properties were added with this method?

For example I extend an adcomputer object using add-member with properties
serialnumber  = 1234
Make = Dell
How can I query the object to see those custom properties that had been added to the object?  

I'd like to write a script will read all added properties and store them.