I didn't see any description stating an effect on spells cast like that bu I know I just flared on the physical plane like a beacon as a Mage. I might still be suffering under the effects of it don't know. I should of known better it always worked for Doc I almost never suffered from physical drain when I overcasted. Electronic dice are my enemy 
From what I recall, when using magic there's no manifestation on the physical plan at all unless it's a "Physical" spell. A good example is the Fireball/Flamethrower Manipulation spells. When you cast fireball, a ball of fire explodes from where you want it to, or for flavor you can "throw" the ball like D&D mages did. Flamethrower looks like flames literally emitting from your hands in a stream, just like if you had a flamethrower.
Another difference is Manabolt/Ball and Powerbolt/Ball. Mana spells only affect living things, so when they go off the living things affected show the effects in the area. Obviously this is kind of boring, or cool, to have a bunch of people fall down for no reason. So you can flavor it all you'd like ("a halo of blue energy erupts from his head and he collapses"). Powerbolt/Ball is a physical spell that produces physical energy. This also affects anything on the physical plane. But again, the spell going off doesn't necessarily have any physical manifestation, but it's very easy to see where it went off since it affects everything in its radius. Essentially, a Powerball will look like a bomb went off, minus the flames. If you've watched "Mythbusters" on TV, think of their slow motion captured explosions. A power ball is like that shockwave that goes off right before the rest of the explosive fire does. In the game, this would smash/crush things, throw bodies from the epicenter of the blast, etc. In my games I always have people roll knock-down tests when they're hit by a powerball, critical failures means they get thrown. Keep in mind that powerball is a combat spell, whereas fireball is a manipulation spell. The difference between the two is that combat spells affect everyone/thing in the radius, and a manipulation spell first manifests into existence then affects the target. So in the case of the Fireball spell, a person behind a car would get to use the car as cover versus the spell. In the same scenario, both the car and the person behind it would take full damage from the powerball.
As far as the caster goes, when they're casing spells you can't tell. Unless of course they have a geas where they need to chant/dance/move their hands, etc. It's described in SR2 that Shamans will have a "mask" that appears as a silhouette that matches their totem, but they don't necessarily have to do anything for the spell to go off.
Again this is all literal "by the rules" descriptions. I like how this works in SR because sometimes you can never tell who the mage is... which is why "geek the mage" is important. I've been wanting to use a mage sniper as a bad guy on my runs for a long time, in essence a mage with a really nice set of binoculars casting manabolts at people. The spells are "Line of Sight" so any optical vision magnification is allowed. How messed up is that?

The offside of this is that magic in SR can be boring to watch since in literal terms you don't really see anything unless you're watching astral. So, flavor as needed for fun and profit!