3M makes an instant lead test product that has great reviews and is under $10. I've linked to the product above.
This is optional but highly recommended; if your house was built before 1978, chances are there are several layers of paint. To ensure you are testing all layers for lead, you'll need to scrape a small amount down to the base layer (plaster, drywall, brick, wood, etc.)
Squeeze and crush the applicator at both the A and B points. This will break the glass capsule contained therein and allow the two testing components to mix.
Gently squeeze and shake the applicator, with the tip pointing down, until yellow dots appear on the applicator tip. Now you're ready for testing.
Rub the applicator tip on the affected area for 30 seconds, being sure that all layers of paint make contact.