I've never had to spot a powerful mage because there aren't any who work out at the gyms I've been to, but I would guess that you'd spot them just the way you spot anybody else. First determine what kind of lifting they're going to do, how much weight they're going to lift, if they're pushing ahead of their usual limitations, and how many reps they plan to do. Then stand in a position that will allow you to carry enough of the weight to help them out if they get stalled, while not being positioned in the way of the lift. Since spotters are most typically needed for the bench press, this is standing behind their head with hands coming under the weight, inside their own grip. A truly effective spotter does more than simply take a bit of the weight to allow the lifter to keep going; they encourage the lifter, and if they have the experience they perhaps note improvements to be made to the lifter's form.

The only thing I'd worry about with a powerful mage is that they might have some self-protection spell that kicks in automatically if they're in danger, and which might be triggered by a sensation of losing control of the weight, and which could knock you away at a critical moment. Just make sure to keep communicating constantly so they don't have the sensation that your focus is not on the task of spotting, and you ought to be good to go.