Once you understand what knit and purl stitches look like, it's a lot easier for you to find mistakes in knitting stitch patterns as you're working. If you know what 2 knit stitches followed by 2 purl stitches are supposed to look like, both in the knit fabric and on your needle, then you'll see mistakes in that pattern more easily.
For example, in the photograph above, you'd be able to see that there's an extra purl stitch. The stitch in the middle of the picture is a purl that should be a knit. You can see the extra bump on the needle as well as in the knit fabric -- there's a bump where it should be smooth.
Once you see the problem, what can you do about it? If you're on the same row where the mistake occurred, you can "tink" (that's knit backward) the row, carefully placing the left-hand needle through the stitch on the row below then pulling out the stitch from the current row until you get back to the mistake.
If you notice the mistake rows later, you can intentionally drop a stitch after knitting over to the stitch with the mistake. Drop the stitch down to the row before the mistake and use a crochet hook to pick the stitch back up, either as a knit or a purl depending on where you are in your pattern.