You measure PO4 bu using a PO4 test kit. Excess algea in your tank is ussually a sign of too much nutrients. This is often due to a high PO4 level. Test your tank water and source water you use for top-offs and water changes.
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian