It has always been a pretty solid bet that an intermittent bird-sized return traveling at Mach 1.2...or even 400 knots is not a bird....to see the smaller signature and discern that it's not a bird and it's not superman so it must be a plane.
As to how well theory translates to reality? well now that both have there own stealth programs they must be at least testing it.
They have always known this.
Problem has always been that such an image is not solid...it is intermittent...and it has been very tough to lock onto for any targeting.