How would they determine if a game is targeted to minors?

This woull be determined by subject matter, visual content, and other indicators similar to those used to determine the applicability of the Children's Online Privacy Protection Act (COPPA).

It also includes games not only aimed to minors:

Games with wider audiences whose developers knowingly allow minor players to engage in microtransactions.

It would prohibit "manipulative design":

Loot Boxes

Microtransactions offering randomized or partially randomized rewards to players.

Pay-to-Win

Manipulation of a game’s progression system –typically by building artificial difficulty or other barriers into game progression– to induce players to spend money on microtransactions to advance through content supposedly available to them at no additional cost.

Pay-to-win - Manipulation of the competitive balance between players of multiplayer games by allowing players who purchase microtransactions competitive advantages over other players.

Also:

These rules would be enforced by the Federal Trade Commission, which would treat the distribution of such games by publishers and online distributors as an unfair trade practice. State attorneys general would also be empowered to file suit to defend the residents of their states.

Source, press release.