Measure and Calculate
Measure the diameter of the dish with the tape measure. If you look down on the dish from above, just measure from one edge of the dish across to the other, right through the center, to get the diameter.
Measure the depth of the dish, which is the height of the dish when viewed from the side. With the dish sitting on a level surface, simply measure with the tape from the outer edge straight downward to the surface it's resting on.
Square the dish's diameter by multiplying the value you measured in Step 1 by itself.
Divide the square of the diameter, the value from the last step, by 16.
Divide the value from the previous step by the depth of the dish you measured in Step 2. This value is the focal length, and it is the distance from the bottom of the dish directly upward to the focal point.
Take your dish outside on a sunny day. You can also perform this experiment indoors if you have a bright, directed light source like a spotlight. In either case, this experiment will only work for a dish that reflects light to some reasonable degree, though it need not be a mirror.
Set the dish down, facing toward the sun or light source.
Hold a piece of paper or something similar above the surface of the dish, directly above the center. If the dish is reflective enough, you will see a spot of light shining on the paper. Slowly move the paper up and down, watching the spot. When the paper is at the focal distance, the spot of light will be at its smallest and most intense. This is the focal point of the mirror.