Strain can be directly observed using e.g. a ruler. Can (internal) stress be directly observed?
Strain is difficult to observe with the naked eye. Many material either plastically deform or break instead of showing visible strain. With the help of polarization filters it is no problem to visualize strain in transparent materials.
This is an image of a plastic ruler under strain viewed through polarization filters:
A high number of bands corresponds to a large strain, areas that only show a single color are less strained. Further details are nicely explained in the wikipedia article on photoelasticity.