How would you represent magic in a painting or drawing?

Nested, rotating and glowing hexagram-like circles on the ground, and levitating debris.

I would add some small sparkles or glitter around and on the object.

It really depends on what the magic does, and whether it’s supposed to have any visible side-effects or not.

If it is supposed to have some visible glowy effects, then you can render those, but I tend to prefer not to want to show magic as “light powers”, as that’s been overdone and does often seem cartoony to me.

Another style is to add subtle outlines of the magic at work, that aren’t supposed to actually be visible, but make the image into a kind of diagram of what’s happening.

Another style is the “halo of arcane symbols” type of thing, which some medieval/Rennaissance Christian artists went in for.

Another style is to not show anything other than the effects, intent and reactions in the gestures and expressions of the people in the scene.

See Kirby dots.

Have a very slim border of white around the hands.

Lightning bolts.

Sparkles like @LuckyGuy stated or halo glowing hands.

I think it would be difficult to do a realistic portrait and just indicate that the hands were magic. I might look at something like Marc Chagall’s paintings and make the whole painting magical realism.

You could put a wand in their hand

What @RedDeerGuy1 said, like the X ray vision spectacles in comic books, only out of the hands instead of the glasses.

Thanks for some great ideas. I think I’m going to try the glow of light around the hands. First I’ll see if I can make it work in a study before I try it on a full-size portrait.

Make them have a sort of multiple outline, so one bold outline, with lesser outlines which are only a hint you can see through.
Maybe a glow, or maybe just break the rule of lighting, so they have no shadowing.

This may be a crazy idea, and might not work at all, but imagine a scene where all objects cast shadows correctly from a light source, except the hands. The hands would have an inversion of the shadow, so where you would expect to have the darkest shading, it would have the brightest highlights. It’s not that they emit light, but that they react to light in the opposite way that one would expect. It might create a really interesting/surreal effect, though I’m not sure if it would be obviously interpreted as magic.

As @LuckyGuy already suggested but I would do that at the end of the spell as something appears. If you want to show the magic in the process I would draw whirling smoke around the area where the magic is going to appear. So if it was a whole room the whirling smoke would take up the whole room. If someone were disappearing then just make them faded with the smoke around them. Almost see-through where you can see furniture through their body and then in the next shot glitter around where they stood.

It’s not an appearing and disappearing act. I want to suggest hands of power without representing a specific action.

Hands emerging from misty background, palms up with levitating object above them.