It's impossible to know which Steam games are DRM-free since Steam games without DRM are not marked in the store. They have to all be assumed to have DRM.
Recent example: https://store.steampowered.com/app/3357650/PRAGMATA/
To the parents question, it is better to use GOG if you care about DRM.
Sometimes the Steam version is qualitatively better because the publisher/dev has supported the Steam version with more updates. Often the updates do turn up on GOG, but it's possible there is a delay.