Can you check air brakes without help?

Easily. I do it every day. For brake shoe thickness, you use a tread depth gauge and, depending on make and model of your truck, you might need to use a tape measure for the steer brakes. Same thing with drum thickness. If the drums are grooved beyond tolerance, you'll be able to feel that grove inside of the brake drums. If you see oil contamination on the brake shoes, there's a good chance you've blown a wheel seal, so you get underneath, shine a flashlight into the brake drums, and see if you can see oil inside of the drums. If you do, then you have a wheel seal which needs replacing.

As for the air brake leakdown test, this is a very basic thing that every CDL driver should have learned in school - if you don't know it, the procedure is listed in several places online.

If you're going to measure brake stroke, a hood prop would be a good thing to have to push on the brake pedal while you're out of the vehicle, as well as a set of wheel chocks - you'll need to have the brakes released while you do this, since the anti-compounding valve will prevent the brake chambers from operating with the spring brakes set, and it'll just dump air out.

While you've got the brake pedal depressed with the hood prop, you'll listen for air leakage... if you hear air leakage whenever the brake control valve and/or trailer supply valve is pushed in, you have a bad emergency chamber, and you'll need to pinch off air lines to the brake chambers in order to determine which one it is - otherwise, the air will just purge out through a purge valve or the anti-compounding valve. If you don't hear air leakage with the valves pushed in, but you do when the brake pedal is depressed, you're losing air out of the service chamber, and you should be able to trace the specific brake chamber by sound.