It takes time to learn LD skills-I would try and make sure you're familiar with well mom and baby, and the most common complications in labour and the first two hours after birth. The rest is hands on skill that comes with practice, especially cervical dilation.
The only tip that I got that worked for me for making sure my exams were accurate was to find my own baseline--for example, my index and middle fingers together are a perfect three cm, and those are the two fingers you'll use to check. I base all me exams off of my perfect three cm--if i cant fit them both in, its a two or less, if i have a little room to wiggle its four, etc. you might find that your two fingers together are a two or two and a half, but then at least you know if it's more or less than what your baseline known value is. Does that make sense?