How can crystal habit be determined when rock is worn and rounded?
Is there a simple instrument


A geological hammer. Seriously. Once you smash a rock, you reveal fresh cleavage planes, grain boundaries, less weathered zones, etc. The downside is that it's destructive.


  angle of a crystal, even if it is worn and rounded?


Here you're assuming that crystals that are not worn or rounded have their original crystal shape. This is rarely the case. These crystals, known as "euhedral" are the exception, not the rule. A good example is feldspar phenocrysts in granites, or some elongated amphiboles. Maybe phenocrysts of pyroxene in basalts. More commonly, crystals are just shapeless, also known as "anhedral".

A more advanced form of rock and mineral study is called petrography - this requires thin sectioning abilities and a petrographical microscope. Basic tools that exist in all geological departments at universities, but probably not accessible to amateurs.