The easiest way to get called a cheater is to basically behave like a blatant cheater. This will also achieve the maximum level of toxicity from the enemy team. Here are a couple things that will make you look like a blatant cheater:

only score headshots
stand on corners uncomfortably long
show your full potential in-game
play without skins
have a low or unreasonably high Steam level
have a private inventory or completely private profile (less fun)
have comments on your steam profile calling you a cheater
use a recently created Steam account
reference popular cheats on your profile (skeet.cc, AimWare, ...)
buy a commend bot so you have an unreasonably high amount of commends
play on common cheater maps (Office, Dust2, ...)
play with common cheater weapons (Dualies, Autosniper, SSG08, ...)
get kills while being flashed
get kills through smokes, walls and boxes
have a Chinese or Cyrillic letter name
have a clan tag that's related to cheating (cheater, semirager, ...)
use a clan tag changer bot (actual cheat with ban risk)
use a kill message bot (actual cheat with ban risk)
be toxic OR don't communicate at all
humiliate "noob" enemies by completely destroying them without showing any mercy