Test if object implements interface
if (object is IBlah)
or
IBlah myTest = originalObject as IBlah
if (myTest != null)
Using the is or as operators is the correct way if you know the interface type at compile time and have an instance of the type you are testing. Something that no one else seems to have mentioned is Type.IsAssignableFrom:
if( typeof(IMyInterface).IsAssignableFrom(someOtherType) )
{
}
I think this is much neater than looking through the array returned by GetInterfaces and has the advantage of working for classes as well.