I am assuming the can has a pressure gauge on it. The easiest way to tell is your system is under or overcharged is by using this chart & calculator. http://www.csgnetwork.com/r134apresstempconv.html
It compares static pressure to ambient temperature. This will give you a general idea if your truck is over, under or perfectly charged. This chart applies to any vehicle using R134A.
Now to test your truck: Let the vehicle sit without running for 1-2 hours. This allows you high & low side line pressures to equalize.
Hook up the pressure gauge & with the engine still off check you static pressure. Note this number & the outside temp.
Go to the chart and find the temperature that matches your ambient temp.
If your pressure reading is higher than the chart then you are overcharged.
If your pressure reading is lower than the chart then you are undercharged.