There are two relatively fast, simple ways to measure a floor for LEVEL (not flat). The techniques depend on what equipment you have.
If you have a rotating laser level (I got one for less than $75 at a big box store), you set the level up in the middle of the room, and proceed to walk around the room with a tape or a graduated rod and make measurements. This will give you a direct way to measure how far out of level any given spot on the floor is. If you do not have a rotating level, you can use a standard builders level on a tripod with a rotating head to get the same result.
If you have a fluid level, or rent one, you can directly measure the elevation of any given spot in the room. Simply walk around, make the measurements, and plot them on a piece of graph paper.
Note that both of these techniques will tell you how far out of level a given spot is. The floor can be uniformly tilted (i.e. perfectly flat), but still be out of level, and these techniques would show an out of level floor (true), but without some math would not show that the floor was flat.