It depends what type of paint it is. Emulsion, watercolour or Acrylic paints can usually be removed by soaking in Water. Oil bssed paints and gloss paints can sometimes be removed with by dabbing with a cloth soaked in white spirits or turps.
Mommy was a hero! My son's shirt had both water-based paint and oil-based wood stain on it!! Mineral spirits worked wonderfully to remove oil-based wood stain from it -- a 100% polyester futbol...