Well directly encountering them is different to one or two being somewhere in the match. In my experience I always see others calling out obvious hacks (like 1-shotting with an impossible loadout at long range), or see a known hacker in the lobby but might not come across them in-game.
Many hacks are also subtle, like damage increase or aimbot, and some hackers toggle them on and off at times to try to be less conspicuous. A hacker might equip bounty hunter but take damage like bodyguard level 3 at close-range, or 1-shot me with an E-11 half way across a map, no beserker/headshot/snipe/explosive etc.
A lot of the time it's not obvious, it just doing subtle things that don't add up, so you have to encounter then a few times to really confirm it.