It depends on the environment, type of plant and time of the year. One only learns through experience, but here are a few guidelines for beginners:

Water one year old or older flower plants, weed-like plants and vegetables in the morning in summertime.  During very hot weather it may be necessary to water them again in the afternoon.  Plants with soft foliage, and that need high humidity (ferns and shade plants), will benefit from having their foliage sprayed with water over noon.  Areas with good, regular rainfall during the rain season need less water as dry areas.  Water can also be given later in the mornings.  Trees and shrubs can be watered at any time, but remember: the most evaporation takes place at noon.