By Colors, Shapes and Content.
A great painting to me has many elements that must come together, composition, light, color, depth, and finally a message
Like it or hate it , its in the eye of the beholder
first by it's impact
It should speak to you.
Art is hard to evaluate because it is individual. But I'll stick my neck out and say that is should be individual and personal and not a random collection of objects in an assemblage - or a pile of soil that would look the same no matter who shoveled it onto the gallery floor.
By the individual- the artist themselves! How did the process make you feel, followed by how it makes others feel. Some may like it some may not but in the end I think it matters to only the artist!
by the onlookers - whoever they are rich, poor, cultured, uncultured, young, old...
We all evaluate art differently. A museum curator will have completely different evaluation criteria than a member of the public at an art show.
As an artist you have to evaluate your own work constantly. I look for composition, color, technical ability and the wow factor.
If I were advising a member of the public what to look for, I'd say "buy what you will enjoy looking at on your own walls".
Buy other Artists and Art Lovers
Not a big fan of "should." If someone were viewing my work, I'd hope that it would inspire the viewer in some way.
By its creativity, how crazy can you get?
If it pleases the viewer, it's good. If the artist says it's art, it is.
That's a complex question.
By the way it "communicates" with the viewer.