If a person with an MG hits you from far away with at least 80% accuracy
They're using no recoil or doing single shots
It takes experience of noticing it. which means it takes time and lots of gameplay.
Agree with you, that´s why ppl sometimes think you using aimbot when you using no recoil. If the enemie is not in movements you can easely kill him with 3 shots using no recoil in case the enemie is not that far from you...