Most Bike Hacks are just workarounds for a tool or a component that you don’t have but probably should. This is not one of them. There is at least one device out there that you could buy right now for $100 which, once calibrated, will project a little red dot directly below the tip of your stem so you know for sure if your bars are aligned or not. But if you’ve got one working eyeball, it’ll save you $100.
In this method, stand over the toptube and position your line of sight so that the edge of the bars that is facing you lines up with the edge of the fork crown that’s facing away from you. Without moving your head, dart your one open eye left and right to the top of each stanchion, looking for an even gap between the bar and crown on each side. Even if your head is a little off-center, it lines up on both sides, you’re straight.
If not, loosen the pinch bolts just enough that you can twist your bars, but you should still need a little force to do it. Instead of pinching the wheel and turning the bars, you can make tiny adjustments by tapping the wheel with your foot while checking your work with the method used above. Once you’re lined up, tighten your stem. Double-check once everything is tight in case the bars twisted while tightening.