Lay the straight edge onto the floor with one end starting at the wall. Straight edges can be purchased or rented from a home improvement store or an equipment rental shop.
Turn on a flashlight and place the light parallel to the straight edge onto the floor. Point the beam of light to the straight edge and examine the floor beneath the straight edge.
Look for points where one unlit section is surrounded by two lighted sections beneath the straight edge. The lighted sections are dips, while the unlighted section between indicates a high point. Using a marker, make a mark on the high points under the straight edge. Most ceramic floor tile installations can handle slight dips with a bit more thinset mortar or a leveling compound filling the lower areas, but high points need to be leveled off to prevent tile damage.
Move the straight edge across the surface of the floor, marking each high point as you encounter one. Perform two passes with the straightedge, with the second pass perpendicular to the first in order to center each high point for later leveling.