Things you can comfortably assume
1. Any line or line segment that is drawn to look straight is straight. This means there is no hidden bend or curve to the figure. It it looks straight, rest assured that it is straight! Be careful, though: this doesn't mean a given line or segment is horizontal, collinear, or anything beyond "straight."
2. If a given figure is named (e.g. Figure A is a polygon) you can assume the figure has all the properties of that type of shape. This means it’s important to know the theorems and the defined properties of shapes. You should know what it means if something is an equilateral triangle or a rhombus, so brush up on your geometry rules!
Things you should never assume
1. A figure is never drawn to scale. If you are not told specifics about lengths, angles, etc. then you cannot make any assumptions about them, no matter how convincing a figure is. Even if a figure looks like a square, if it isn't called as square, you can't say it is one. You need to know about its sides, angles, whether the sides are parallel, etc. Many fall into this trap, so don't be among them!
2. Don't eyeball any measurements for lengths or angles, either. You cannot assume right angles, for example, unless the diagram is marked with the perpendicular square symbol in the corner or you are told that the figure is a specific shape that has right angles. Likewise, lengths may not be the same and lines may not be parallel, even if they look that way at a glance.