How can you tell if a physical change has occurred in a rock?

by its shape and size

Take into consideration, what you want to notice. It would help if you take pictures, measurements, and detailed description of the rock you take. Take into consideration, that recording the dates of these would help.

Say you have a square cube in water, if you measured it today, write it down in a notebook. Take pictures as well, and create a scale of it. A couple weeks later, it may have shrunken down a bit. Measure it then and compare the size in between the two dates.

"Physical change" can take place in many ways. The evolution of a rock can most often be read by colors and seam lines.