I wanted to return to SpaceChem, but the forum said it lacks updates compared to the steam version. Is there any way to check this?

Only way besides looking individually is a dot in the game library. As in, buying the game. Putting it in your library. When an update takes place, you will see a colored dot on the title icon. About all there is.

That is why there are threads in the forum that tell people what updates happen on the site.

The version on GOG through Galaxy is v1016, which is on par with Steam, and has been for a year. For whatever reason, GOG never updated the offline builds.
On Galaxy the only way you can check is, when auto-updating is disabled, by right-clicking on the game and selecting Manage Installation -> Configure. You will see a list of versions you can select, and next to the version number is how long ago it was uploaded.

Thanks, I thought it was something like that, but couldn't quite remember. Guess I could download Galaxy and get a fresher version, but nope! I'll just find something else to play.

Check randomly and be pleasantly surprised.