Hi and welcome!
You should be able to lay the scarf out to see where you're increasing stitches. Inadvertantly moving the yarn over the needle when you switch between knits and purls will leave a hole that you should be able to see if you stretch the scarf out a bit widthwise. If you're adding stitches at the end of the row, that should show up too because the scarf will be skewed at one or both edges. You can fix that by looking at the edge stitch and pulling the yarn down from the needle before knitting rather than pulling it up and over the needle (the latter makes one stitch look like two).