Detecting semen can be done a few ways. Online kits use a liquid to expose semen as a different colour. Although these kits are accurate, they can be expensive, especially if you are testing multiple articles of clothing. Each kit yields results for five to 10 articles of clothing. A less-expensive but still accurate method of detecting sperm is a black light. A black light uses ultraviolet rays to detect fluorescent materials, including sperm.
Buy a black light. You can find relatively cheap black lights at most department or home supply stores. You should be able to find a black light for £3 to £6.
Gather the clothes that you will be investigating. Look for stains on them that you can see with your naked eye first to determine which spots you should take a closer look at.
Scan closely the clothes you have gathered. Hold the light about six inches from the clothing and move it slowly across the piece entirely, focusing especially on the spots that seemed to be present before to your naked eye.
Observe the clothing. Semen will show up as a light yellow colour when scanned with a black light. This should give you an answer as to whether semen is on clothing or not.
This should only be tried on clothes that have been washed, as semen will remain on materials that have been washed. Substances that are oil-based can show up under a black light as the same pale-yellow colour, so make sure the clothes have been washed at least once if you want to be sure.