Rabbi Falk (Gateshead) has a booklet called “Madrich l’bedikat tolaim”.
He goes through all the vegetables and fruits and explains how to check them.
He explains that for broccoli, it’s easiest to remove the florets (to discard) and just rinse and inspect the stems.
The florets can be checked, but insects are harder to see because they tend to be darker in color, closely resembling the color of the broccoli. He recommends using a white or light yellow bowl, filling it with warm water (which causes the insects to loosen their grip) without salt. He says salt clouds the water.
Dip the broccoli in the water and run your thumb over some of the florets to loosen them. Swirl the broccoli in the water vigorously a few times, then remove and shake it over the water like a thermometer. Check the water surface after half a minute and remove anything suspicious onto a white napkin or white paper plate. Next, inspect the bottom the same way after pouring off a bit of the water.
The broccoli is edible if you find nothing.
Cauliflower is considered easier as infestation is less common. Cut off the bottom primary stem and accompanying leaves. Cut the cauliflower into florets and cut those in half, bending them a bit so you can see clearly. Soak it in salt water as a precaution (and check the surface of the water). It can be used if no insects are found. He says to discard the top of the stems if you find a green fly or thrips as it would be impossible to check the rest carefully enough for further infestation. Black flies (up to two on a head) are not considered problematic. You remove them and do what was written above to make sure all is clear (I actually look at the head wrapped in plastic before I buy. I have sometimes seen a fly on top of the head just inside the plastic wrap).