How to check in NAnt script whether property is set or not?

Hi I am create a release script using NAnt. How can I check a variable value is getting or not.

Now I call my script like this

    nant -buildfile:CreateNew.build -D:name="Test.V.1.0" -D:bIDs="2" -D:uIDs="'3'" 

Some times I will not pass uIDs.

So I need to check in my nant script whether the uIDs is getting or not. How can I do that?