We sue a combination of indicators.
I like the pelvic one, we have two at 2 fingers width..patiently waiting lol. 1 finger wide 1+month, 1.5, 2-4 weeks 2+ any day now...well that's the theory. We also have a RIR at two fingers, but tiny light pink comb.
Other signs include;
Big comb and wattled,
red faced, combs wattles and ears,
wide level stance,
vocal / 'egg song'
'squatting'
interest in nest boxes.
I think 20-22 weeks is 'average', but like us they mature at different rates, some earlier some a little longer.