UP!'s software has a handy little feature that makes it easier to tell if your platform is level. The Maintenance dialogue allows you to raise the platform up to the nozzle, which remains at a constant height during printing. From there, you can move the platform to check distance from the nozzle for all four corners and the center. You can then adjust the platform upward in 100 micron increments, sliding a folded piece of paper back and forth between the nozzle and platform until you feel a little resistance. At that point, you can back the platform down from the nozzle a hundred microns and if you feel the paper become loose again, you know you're in the 200 micron neighborhood.
But, what if the center and four corners are not all the same distance from the build platform to the nozzle? Bad news. That means your build surface is either warped or not level. If, for example, the right rear corner of the platform is a half millimeter from the nozzle, the center of the platform is a millimeter from the nozzle and the left front corner of the platform is half a millimeter from the nozzle, you have a warped surface resembling a taco shell. If the surface is not level, whichever corner is the opposite of the corner that is too low would be too high by the same amount and the center distance will be a mathematical average of those measurements.
Though a warped build platform is a rarity, an uneven surface is not. Almost nobody attempts to print directly on the build platform. We cover it with blue painter's tape, Kapton tape, perforated fiberglass or Pyrex glass with something like hairspray sprayed on it so the plastic sticks to the surface during printing. If your 3D printer's build platform isn't warped, it is likely to seem warped once your surface is prepared in one of these ways.
If the platform or prepared surface isn't level, there are going to be screws underneath the platform to adjust it. This could take a while and it isn't likely to make it onto your list of fun things to do in your spare time. You will get better at it with practice. However, there are no screws to fix a warp. If the warp is in the surface you prepared, you can prepare the surface again to try to solve it. But, a warp in the build platform itself is the great white shark of 3D printer doom.
Until recently, this meant sending your printer back to the manufacturer for repairs, or attempting to take the machine apart and replacing the platform yourself. If personal 3D printing is ever going to expand well beyond the maker community, this is not something that can be asked of those who purchase the devices. Even turning screws to level a platform is too much to ask, as is proven by the high end professional machines that automatically level themselves or require no leveling at all. This technology will eventually make its way to less expensive printers, but for now, it is outside the personal/prosumer price range. So, when a prosumer 3D printer manufacturer does something to address the issue of warped build surfaces, it merits attention.