Tensile strength is measured by a tensile testing machine with load cell. A specimen is loaded to failure and the load divided by the specimen cross section area is the tensile strength.
I am not sure what you mean by displacement. If you know displacement under tension and the specimen length, then
Y = PL/AE = tensile strength X L/E where E is moduklusof elasticity. Butthis only works if linear, like to the yield point, but not to ultimate (material would be in plastic zone after yield) , so you can only get tensile yield strength, unless the material has no yield (like glass) .
The displacement would also have to be measured with strain gage for accuracy, and it strain you are measuring ( displacement/length).
Ordinarilt .t he strenght is