The way to know is when ASX put up an exact time of day on that "upcoming ASX floats & listings page" (usually IPOS list at 11:00am) on the "proposed listing date" column...
having said that, I have noticed probably one or two times over the last year when the person at the ASX who is supposed to update that webpage has not done so.. (lucky the ASX isn't a multi billion dollar listed company otherwise that it would be pretty pathetic right?)...
However the safest bet if you don't trust the website guy/girl at the ASX is by checking the ticker each day around 4pm and you will see a tonne of pre-listing disclosure uploaded the day before it is actually due to list...
GMV Price at posting:
0.0¢ Sentiment: Buy Disclosure: Held