Ready to do some painting over old paint? Wondering whether you need a latex or oil-based paint? If you are unsure of which type to buy, here's how to tell (as easy as pie). From deep within the Carey brothers' vault of test-and-tell magic, we bring this surefire, foolproof ?smear? tactic. First scrub a small area with a solution of houshold detergent and warm water. Rinse well and towel-dry it, then soak a cotton ball, Q-Tip or soft rag in alcohol and rub it back and forth over the cleaned area. If paint comes off, it's latex -- and another coat of the same is in order. But, if the paint doesn't come off, it's oil-based, and a pre-coat of oil-based primer is a must. Then you have options for the finish coat, as different types of latex or oil-based paint can be used. At this point, after testing and priming (if needed), you might want to discuss both your options and paint performance characteristics with the paint experts at your local hardware store. And that's your On The House Tip for today.