Ketone urine-testing strips, also called Ketostix or just ketone sticks ... are small plastic strips that have a little absorptive pad on the end. This contains a special chemical that will change colour in the presence of ketones in the urine. The strips may change varying shades of pink to purple, or may not change colour at all. The container will have a scale on the label, with blocks of colour for you to compare the strip after a certain time lapse, usually 15 seconds. Most folks simply hold a strip in the flow of urine. However, the force of the flow can "wash" some of the chemical away, experts advise that a sample of urine be obtained in a cup or other container, then the strip dipped into it.
The chemical reagent is very sensitive to moisture, including what's in the air. It's important to keep the lid of the container tightly closed at all times, except for when you're getting a strip to take a reading. Make sure your fingers are dry before you go digging in! They also have an expiry date, so make note of this when you purchase the strips ... that's for the UNopened package. Once opened, they have a shelf-life of about 6 months -- you may wish to write the date you opened on the label for future reference.
Ketone test strips can be purchased at any pharmacy, and are usually kept with the diabetic supplies. In some stores they're kept behind the counter, so if you don't see them on the shelf, just ask the pharmacist; you don't need a prescription to buy them.