There are a couple of ways to “know which heroes are good tanks” etc.
- There is a positioning guide that lets you know the optimal and sub-optimal positions for each hero
5* heroes positioning guide - Anchor Grading also rates heroes based on titan, offense, and defense. In the third category, there’s a “tank” rating
AnchorGamed.com Hero Grades - Google Sheets - Experience
The third one takes a while to obtain, but the first two are much easier to access.