I experienced the same problems at school. It's definitely a mix of being able to judge whether an answer is sensible, taking your time when going through the question, checking your working out (approach it as if you are marking it and try to forget your method), and if you have time re-doing the question or trying a different method. If you do try a different method, you might pick a method that takes a lot longer and might waste time, so look out for that.
I think the biggest problem I had was that I would instantly launch into the questions and try to finish them as soon as possible. You need to slow down and take care in how you write out your answers. Lay them out in a structured way so that the examiner is absolutely clear on your working. This will not only make it easier to check, it will also slow you down and make you think more about your working. More importantly, it is good practice for if you go on to do maths at university, where it is crucial to structure your answers cohesively.
Another tip is to space your working out a bit and to write neatly, so that you can make adjustments to your answers without having to re-write it all.