How can a homeowner check for low freon in air cond?

place system into call for cooling at stat, then head out side and grab ahold of larger of the 2 copper tubing pipes that connect indoor coil with outdoor unit. this pipe should be cold also on days where it is hot inside the smaller line will be hot so be careful. other indicators are a noticeable shift in humidity within the home even if tstat displays set temp, frost will form on line from indoor coil to outdoor unit. there maybe water on floor. baring this levels must be measured with a gage