Question

How do I know that I have been understood? If not, what do I do about it?

Answer 1

In what sense are you asking to be understood?
My take at this question would be that whoever you are communicating with should be able to rephrase and repeat what you said back.

Answer 2

You can ask them
And if they don't understand, you have three options:
1. Explain to them again/differently
2. Ask them what they didn't get specifically
3. Give up