Load weigh systems.
Earthmoving trucks have had loadweigh systems for quite a while.
Pressure transducers in the struts and a control (summing) module - not such a great expense.
Not sure that accuracy would be suitable for more than a cross-check, but the hard part for the trucks is the slope and condition of the ground that they're sitting on.
That may not be a problem for aircraft on the apron. Or I hope not.