With your trailer loaded and hitched up, pull just your vehicle onto a commecial scale, making sure the trailer is not on the scale. Weigh your vehicle with the trailer attached and note that weight.
Next, unhook the trailer and jack up the trailer until there is no trailer weight on the hitch. Make sure the trailer jack is not on the scale. Record that weight of only the truck. This is your Gross Vehicle Weight (GVW).
Now subtract your GVW from the weight with the trailer attached. This will give you your Vertical Tow Weight