How do you know if paint is latex or oil based if its already painted?

I need a cheap quick way to find out if already painted-on paint is latex or oil based.How do you know if paint is latex or oil based if its already painted?
Shouldn't matter. Either one can be repainted with latex.





One trick that MAY work is latex can't be sanded very well. It's really elastic, and will clog the sandpaper fast. Oil is a solid surface, and can be sanded.How do you know if paint is latex or oil based if its already painted?
If you don't know exactly what it is, the best thing to do when getting ready to repaint is to use a primer. Better to invest the money in the primer than have to paint it twice and end up with getting primer anyhow.