"Sillage monster" usually refers to a perfume that is known to put out very heavy sillage.
What you describe is more like a sillage slayer.
Anyway, to answer your question "how can one tell?" I'm not sure. I haven't figured out how to accurately judge the amount of sillage my frag really puts out. I can only really tell the difference between a sillage monster and.....one that is not.
One of my sillage monsters - JHL - I can be sitting still even hours after applying this stuff and easily smell it wafting up from my wrists.
One of my not so sillage monsters - Idole de Lubin - after a couple hours, I have to sniff my wrist pretty close to be able to smell this one.
Maybe some other members have some other/better techniques for sillage testing.