With the use of a Urine-Off™ Black Light, various bodily fluids that are not visible with the naked eye will fluoresce, indicating where these fluids have been deposited. With better visibility, these surfaces can now be properly cleaned with Urine- Off. Light only. Urine-Off sold separately.
 
The crystalline structure of dried urine causes it to fluoresce a dull yellow color under ultraviolet (UV or "black") light. Thus, a Black Light becomes a valuable tool for locating the exact areas that need Urine Off.
A few notes on Black Lights:
Incandescent bulbs that screw in to a regular lamp socket do not work; a fluorescent tube-type or LED light is required.

"UV" refers to a specific band of light wavelength. Generally, urine will fluoresce under any UV light - but some exact wavelengths do a better job of fluorescing urine. The Urine Off product line of black lights employs custom circuitry to produce the optimal light wavelength for urine fluorescence, and so work better - which means you don't have to have the room perfectly dark, or hold the light source so close to the surface. Hardware-store black lights work okay, but are harder to use effectively for these reasons.

Urine is not the only substance that will fluoresce. Some soaps and detergents have components that fluoresce, even after rinsing thoroughly, as well as some fabrics and lint. These will usually fluoresce a bright white or even blue-ish hue, not the dull yellow of urine.
Use protective eyewear: Prolonged and direct exposure to UV light can be harmful to the eyes. The ideal type of protective eyewear is safety goggles that can be purchased at any local hardware store.  These glasses, like most eyewear made today, filter UV rays.

Take the black light into the bathroom and darken the room. Inspect the toilet, especially around the seat hinges and the outside front of the toilet bowl. This will allow you to see exactly what urine looks like under black light. Even the cleanest bathroom will show glowing yellow urine crystals, because ordinary cleaners simply do not dissolve them.
Close the curtains/blinds if necessary to make your work area easy to see under the black light. 
Once you've located all the affected areas, you might want to mark them with a piece of masking tape so you can find them when the house lights are on.
NOTE: Urine Off's bio-enzymatic action is capable of breaking down the uric crystals completely so that their fluorescence is eliminated, particularly on hard surfaces like porcelain and glazed tile. The initial buildup will likely take 2 or 3 treatments, but light buildup will need only one treatment. Porous surfaces like fabrics, carpet, and even concrete, can retain so much residue that complete elimination of all fluorescence may require diligent extraction efforts with commercial equipment. This is simply too costly - once the urine stain and odor is removed, the goal has been reached!