If you have a scanner that is 3 years old or newer, you can scan them and use the scanning parameters to make the receipt more readable. I use this method a lot for my tax clients.
The problem with receipts is that they are printed on thermal paper, which causes the ink to fade or the ink in the paper to become overexposed to blacken or gray out the receipt.
You can also use a hair dryer at a distance of about 12 inches on medium heat to make the images reappear. Try this only if you are unable to get the scanner to enhance the image.
I do not know the CSI episode to which you refer. THese shows over utilize technology that makes it appear very simple. How wonderful it would be if there were a chemical I could just spray on the faded document and it becomes visible again. Most sprays used in "CSI" are reactives that detect certain kinds of chemicals or imprints, such as chemicals in the blood (hemoglobin which is iron based), and so forth. The chemical in question would be reactive some older kinds of inks found in printers and such, but not necessarily appearing on heat sensitive paper such as used in the receipts.
There is no spray on chemical that can cause receipts printed on heat based ink paper to reveal the faded letters.
The process used is the scanner method or an imaging software.
As long as you have the original, not a picture or copy, you can scan the faded image in high def, not JPEG, and when it comes on screen, you play with the lighting and filters to see a darker image, even turn it into a highlighted negative to see it better.
The problem with CSI, is that they sometimes use "movie" staging techniques which, while having an apparent scientific foundation, are not possible in all situations. So the viewing public gets the idea, this is real. Remember, it is still Hollywood.
If a product were available to do this, it would be heavily marketed in office supply stores everywhere. This is because after six months, all receipts from stores like office supply, lowes, home depot, etc fade. AND those receipts are needed for taxes, refunds, and income statements, etc.
For those inks that a reagent would work, the reagents are hazardous chemicals, and are not normally purchasable by the general population. A photo lab that still processes photos by hand, may have the reagents that can make this possible. If they are willing to share some with you, you can try it.
from my experience the use of gentian violet does sometimes bring up faded receipts. Use with caution. Most Pharmacies will carry a 1 percent solution. Place in solution until it starts to darken. Pull out and place in standard alcohol and use a standard black light to enhance the writing.