Read the settings (settings before joining) and look at the history (history once the game has ended).
If there's a custom scenario, make sure to view it before joining. If they're attacking multiple places at once, it's a feature called muti-attack (see https://www.warzone.com/wiki/Multi_Attack
It's impossible to cheat, unless someone uses multiple accounts in a way that gives them an advantage. E.g. alternative accounts in different teams which are all operated by the same person.
Fell free to report using the in-game report button if in doubt.