It's not hard, or expensive.
In the early 1980s I designed a system, for a major UK articulated lorry fleet operator (some 200 lorries IIRC) who wanted to load them to the limit without getting busted on random weighbridge checks, on total weight or axle weight.
It involved fitting strain gauges to the leaf springs but they could go in other places, near the suspension mounting points.
Production implementation weas handed over to somebody and I don't know how far they got, but I remember that many patents had existed on these systems even back then.
On a large jet, the simplest way would be to measure the compressive strain of the steel of each landing gear centre leg. Very easy with a strain gauge, and accurate to maybe 1%.
Obviously it would only work at standstill.