There is a common saying here on PC and everyone will agree to it. "USE THE REFERENCE." <.< That applies to the texture as well. For example: If you are creating a rock, and you have unwrapped it. Use the real life rock and see how you are implementing texture. =\ Creating a rock is very basic so I suggest you should…
Mind showing us an image of what you're trying to achieve and what you mean? We can guide you better if you have some example, also, have you started a Pimp and Preview thread with your stuff? That can help.
I made a UV grid/checker texture that helps with this. It has text that makes the mirroring more obvious. The color gradient makes tiling obvious. The checkers and circles make distortion obvious. Circles are especially helpful for seeing distortion, since the human brain seems to be adept at judging the proportions of…