Hello Elizabeth!
Here are some few things you can't assume from a diagram.
1) No diagram is drawn to scale.
2)If angles appear to be right angles, you can’t assume they are right angles unless the test says so, or unless a little “perpendicular square” appears in the diagram.
3) If lines appear parallel or perpendicular, you cannot assume either if it’s not specified.
4) If lengths appear equal, they may not be.
Best of luck with your studies!