Best way is to use a trigger pull gauge.
Other than that try and use a fish scale, or take and put together enought lead weight till it will pull the trigger.
This will give you roughly the trigger pull weight.
Hope this helps.
Fish scale and a bent coathanger seem to work well here. For the light ones I've got an antique 2 lb. spring scale that will get them down in the ounces. Really the best way is what feels good to your finger though not what a scale says.