So you can use Archimedes principle to find the volume of helium in the balloon. You just need a sensitive enough scale. The bouyant force on the helium balloon is equal to the weight of the air it displaces.
Fb=da * g * V,
where da is the density of air and V is the volume of the balloon. Now tie the balloon to an object, something like a wood block, whose weight you know to be w1. Next weigh the block with the balloon tied to it. The weight registered on the scale now will be the weight of the block minus the buoyant force on the balloon.
w2 = w1 -Fb
w2 = w1 - da * g * V
V = (w1 - w2) / da / g
So now you know the volume of helium in the balloon. The mass of the helium is thus
m = V * dh.
m = (w1 - w2) * (dh / da) / g
Or in other words. If you dont have the right instruments you can forget it to check