If it's a dry oil based paint, then you'd need to use a paint stripper.
If it's an emulsion paint, then it's much easier to remove, and any strong solvent will dissolve it. If it's not a large area, then just use some nail polish remover to take it off. If you run out of that, even rubbing alcohol (isopropyl alcohol) will dissolve it. If it's a large area, then I'd buy some lacquer thinner or xylene and dissolve the emulsion with that.
Also, as stated in a previous post, try picking it off with the corner of a razor blade or with the point of a sharp knife. It may come off quite easily.
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