I completely agree with Patrick's answer, but would like to add that it is indeed possible to have immediate scroll events on the mobile versions of chrome and safari, you are talking about.
The issue you're describing is related to all iOS devices before version 8.
Before then the mobile browser engine did not trigger "true" scroll events while scrolling, but only once scrolling had come to a full halt (so after the inertia stopped). Even the execution of javascript was suspended, so you couldn't just run a loop to check the scroll position.
Long story short: There are ways around it using containers for scrolling and requestAnimationFrame
.
The easiest way I found to get scroll events for pre iOS8 devices is iScroll.
I would recommend to check out this: http://janpaepke.github.io/ScrollMagic/examples/advanced/mobile_basic.html
and this:
http://janpaepke.github.io/ScrollMagic/examples/expert/mobile_advanced.html
So getting back to your original question: How to detect when you would even need those workarounds.
You'd need to check if the you're on iOS version 7 or lower, which, to my knowledge, can't be done with modernizr, as it is a feature detection library.
So check out this thread to learn more: Detect iOS version less than 5 with JavaScript