The non-precise way you've already heard about, sheet of paper, check for dragging.

The better (more precise) way, is using a magnetic base micrometer attached to your extruder head. You can then watch the bed run-out in real time; if your 3d printer supports many configurable sections across the bed ( I know Prusa style cartesian units mostly all support quadrants) you can record the run out everywhere across the plane without any more physical work than watching the gauge and recording the results.

P.S. be careful using both the micrometer and the feeler gauges on a 3d printer bed. Most work plates now-a-days are using PEI coatings that'll scrape off easily with metal-on-metal contact.

Yeah, you're correct, it's a glass bed so I use it to do the 4 corners (every few months) and then use a BL touch to do a 3x3 grid on top of it.