Heavily infested branches will drop their needles. Close examination of infested foliage may reveal very fine webbing on the stems at the base of the needles. Spider mites are tiny and difficult to see with the naked eye on the plant. A convenient detection technique is to hold a sheet of white paper under a branch and then shake or tap the branch against the paper. The mites, if present, will show up as tiny, slow-moving specks on the paper.
Insecticidal soap is available from retail stores under various trade names.
You may substitute ordinary dishwashing detergent for the commercial products with little risk of injury to trees. To make your own insecticidal soap spray solution, mix 5 tablespoons of liquid dishwashing detergent in 1 gallon of water. Soap sprays have no residual activity and only control mites and insects that are contacted directly. Thorough spraying is important for control. Use the garden hose to apply a forceful stream of water to dislodge mites from an infested plant.