One of the more common things I hear from people who lack confidence in their knitting and crocheting is that they don't know how to spot mistakes until they are nearly finished with their project, or, even then, they can't see what they did wrong and only know that something isn't right. And, they often lament people who are able to immediately spot and fix mistakes, sometimes without needing to frog entire projects.
The truth? It isn't magic. People who can spot and fix mistakes without needing to completely start over or who can tink or rip their crochet out to where the mistake was made are able to do so because they can read their stitching.
Yep. That's it. They read their stitching.
Like any written language--English, French, Spanish, Japanese, computer--stitching does have its own grammatical system, if you will. A knit stitch is only a knit stitch. A single crochet also has a particular look.
Now, how you get there may vary, but the resulting stitch--whether its a right-leaning 2x2 cable or a half-double-decrease crochet--will have the same features, and once you learn to recognize those features of the individual stitch, you'll be able to read your stitching.
This means that as you follow along a row or column of stitches, you'll be able to count your stitches more easily, figure out your gauge more quickly, spot what you did in terms of the pattern and find where you left off, and, yes, know where your mistakes are. Moreover, once you understand the structure of the stitch, you will be able to more easily fix that stitch without needing to start all over again.
There are so many online resources right now to help you learn to read your stitching. I highly recommend Little NutMeg Productions tutorials and articles about reading your knitting. As for crochet, a good beginner book, like Susan Bates' Beginner Crochet or even Debbie Stoller's The Happy Hooker are also incredibly helpful.