It depends what was used to stick the labels on in the first place, obviously not something that was water-soluble, like in the good old days. Alcohol and petrol are both good choices, but both flammable and should be used in the open air to avoid being affected by fumes. Depending on where you are alcohol = methylated spirits or denatured alcohol, and, if you can get them, those little plastic 'lighter fluid' containers for the petrol (petroleum, gasoline). You may have to scrape at some of the label to allow the solvent to get to the glue.