Most helicopter pilots (but not all I admit) work using the limits, charts and whatever is available including experience to stay within the legal, safety limits of the aircraft and it is actually fairly easy to do AS LONG AS you know what's ahead of you, Like Geoffer said "to look at the mass situation for your flight BEFORE you get airborne" That's all nice but it is NOT always the case. Certain operations demands improvisations and guesses on what is ahead of you. Therefore sometimes on a windy day you may end up picking up a load that would have stayed on the ground the day before. Logging is a quick example of this, you never know what your next load is going to be even if they give you a "guesstimate" on the next log.
With a weigh scale on board or using known power settings etc a pilot should be able to figure out the weight he is picking up especially if he is experienced on the aircraft he is flying.
Also as a note here, wind will affect OGE hover weight and that can be either way Up or Down, the UP is the easy side of it.
JD