How do you test a painted surface to determine if the paint is oil-based or water-based?

If the previously applied paint is oil-based, I know that water-based paint will not properly adhere to the surface. The painted surface is in good shape, so I do not want to apply a primer (I am replacing a few boards of exterior siding, and i need to apply the new paint, which was color-matched to the original paint, to previously painted boards adjacent to the new siding for blending and nail hold repairs). The previously applied paint is of unknown origin or manufacture.How do you test a painted surface to determine if the paint is oil-based or water-based?
you can use Goof Off or Oops brand of solvents, they remove water base and not oil base. clean rag-oil base/dirty rag-water base.Now lets address the facts about changing top coats. On exteriors never put oil base over latex .you can however put latex over oil base. Water base paints will adhere to oil base paints as long as it is not glossy. If its glossy then sand or use a primer formulated for glossy surfaces. When in doubt prime. If the job is worth doing then error on the side of cautionHow do you test a painted surface to determine if the paint is oil-based or water-based?
Thanks to everyone for taking the time to answer my question. Recommendations were great: I used the alcohol test, and confirmed the original paint was water-based (acrylic). Thanks again for the help!

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take a tissue and rubbing alcohol or nail polish remover (make sure it is acetone based.) rub on a spot if the paint is removed it is water based. If you really are not sure there are kits you can purchase to do the same job....you can paint oil over water or oil providing the paint you painting over is not a gloss paint... but it is still best if you primed first to make sure you are getting the best possible out come.... as for the paint you use.... You get what you paid for... cheap paints are not the cheapest since you will be repainting it more often then a good paint which often have guarantees.....
use rubbing alcohol. Put some on a rag and lightly rub the surface. If the paint comes off on the rag, then its water-based.
If your old paint is oil based it should have a slight film on it, like dust, if it is old. Rule of thumb is you can put oil over water based, but shouldn't put water based over oil. It will hold for a while but will let go at some time in the future. If you're not sure and your new paint is water based, add an additive to it...like Flotrol...it will help it adhere better.