Regarding information about wheel wobble I have some of those old fashioned mag wheels on a fixed-gear bike. The back one seems to wobble about like mad when riding. A random stranger even pointed out that he could see it wobbling about like mad. Depending on what tire I am using, I can feel it moving about like mad. At the minute I am using a 700x28 Tannus tire. Just wondering what this could be? Weak wheel? Buckled? If so how would you adjust it as it doesn't have standard spokes
There are two possibilities:

1) The wheel is moving at the hub because its bearings are failing or need adjustment. Lift the rear of the bike off the ground, grab the wheel rim and try to move it from side to side. If it can be moved, the bearings need to be looked at.

2) If the rim cannot be moved side to side, spin the rear wheel and look at where the rim passes the brake pads. If the passes closer to one or other of the pads as the wheel spins then the rim is bent.

If the rim is bent then I suspect it cannot be straightened on this type of wheel.

Some wheels have hubs with adjustable bearings. Without knowing what exact wheel you have it's impossible to say if yours can be adjusted. If the bearings have failed then the wheel may be unrepairable.