Gas bag
Most common is a strain gauge. If you can imagine a little stamp self adhesive that has like a "S" in the centre of it with wires to each end. As the material stretches under load the "S" also stretches and they can measure the resistance across the "S" this is recorded and then they can work out the load on the part. They put 100s on when testing.
With computers they can also do modelling with program's. But still sometimes you only have to use your own eyes to know what's going to work and what's not.
White man magic
Cheers