no, it’s not a reliable indicator because the bleeding on your break occurs due to the drop in hormones, not because an egg wasn’t fertilised. please re read my original comment. any bleeding on your break is a withdrawal bleed. implantation is rare and might not even be a thing and it’s impossible on the pill as you don’t ovulate. i would only be concerned about pregnancy if you’ve missed pills. the pill works by thinning the uterine lining too which is why you might stop bleeding or only have light bleeding on your break and that doesn’t mean you’re pregnant.
if you missed pills and have heavy bleeding then yeah sure use it as an indicator because a pregnancy would not be viable but the best indicator is taking your pill correctly as the bleeding on the pill is either a side effect (breakthrough bleeding during active pills) or due to the drop in hormones (withdrawal bleeding during your break)