A rocks texture cannot be the sole determining factor in rock type identification. If the grains are loosely cemented and come off in your hands when you vigorously rub the rock, there's a good chance it is sedimentary, however.
The most obvious way to tell whether or not a rock is sedimentary is to look at its texture. Remember, sedimentary rocks are formed when sediments, fossils, shells, etc. are weathered and then...
The simplest way to tell if it is sedimentry is to look for layers. In shale it is very simple to see them. If you know the location of where you found it that can help too. Alot of sedimentary rock...
igneous rocks usually contain grains of crystals, like granite.sedimentary rocks sometimes have stratification or fossil remains, for example, limestone.