We have to say that it's more an issue on older motorcycles, a quick check of a few less traveled models showed that those wee markings on the swingarm adjuster aren't necessarily gospel. In fact, some of them are downright liars.
With a quick check, you may find your wheel alignment is correct, as was the case with the last motorcycle we checked. If not, the old way still works if you don't happen to have any lasers in your toolbox. If your motorcycle feels better carving left than heading right, or the otherway around, it may just be this simple. Be aware though: if you've purchased a used motorcycle and the frame and/or swingarm is bent, it will really stand out.
Although technically the stringline will be accurate if under enough tension, make it easy for your self and use a well-swept concrete floor. Right, after that you ideally need a race stand to give a clear line for the string to go through. You'll also need a couple of relatively heavy objects to hold tension on the string. Axle stands or fuel containers – full, obviously – are fine if you have some around. Slide the stand back far enough to have a little weight on the front tire, so it stays put. That's if, like us, you have no friends to hold the handlebars straight.
With the string wrapped around your rear tire and couple of times to stop it slipping, try to get the line as high up off the ground as possible. This is to give as wide a spread as it crosses the rear tire edges as possible. The larger the spread, the more accurate it'll be, as the string extrapolates the line out in front of the motorcycle to the weighted tie-point – the axle stands.
With the string just toughing the front contact of the rear wheel, you now have lines indicating the rear wheel's alignment in the swingarm. The next step is to get your front wheel perfectly straight, it is from center. This is the spooky bit as it may be obvious that your wheels don't follow the same path. Don't despair, at this stage it may simply be the little calibration marks were stamped out with little regard to accuracy. If your motorcycle is from the early '80s. It's almost a given that this is the case. Maybe some staff was using Imperial units while the other side of the motorcycle was metric. But rarely will you find one of these trolleys with accurate adjustment marks.
Back to the task, if you loosen the adjusters off and with the help of a rubber mallet or a firm hand, you can gently steer the lines to such a place that your front wheel is central to the strings. This is where your chassis ought to have everything in line, as your wheels now are.
Ideally, if you do a mini version of the stringlines on your sprockets, they'll coincide and prove that your chain is also nice and straight, adding life and minimizing power loss through friction of a bending chain. The flip side is that a bent frame will not let you get both the wheels and chain to agree on an ideal path…