How about asking the person to reflect what you just told them and see if it matches what you said?
I'll often ask people (especially family members), "Am I making sense? Do you see what I'm saying? I'm not sure if I'm explaining myself the way I'm wanting to, can you tell me what you got from that?"
I try to stay away from insinuating that they just don't understand me. That just leads to frustration on both parts. I'll take responsibility for the miscommunication first, that way they can see that I'm not attacking them.
(I'm not implying that you do this, just explaining my whole process.)
Then, I'll keep trying different methods of communicating the same message until I get clear with them. Especially, if they're starting to give up on trying to understand me.
A lot of times, I'll try to 'enter their world'. I kind of detach from myself and think okay I live in Bobville or Susanville now, how do these people talk? Are there any analogies I can use to make this clear? Are there any hobbies/sports I can relate this picture to? How does this person think and how would I react to my message if I were this person?
The best clue is how they express themselves. What sense words are they saying? Are they feeling, seeing, understanding, hearing you? Be a translator. How would you translate to someone in a totally different country with a totally different language? Sometimes we're speaking the same language but still need some translating.
If they completely miss my point I move on and try something else. I see a lot of people trying to repeat themselves over and over the same way to no avail. Going back to the different languages analogy, that's like repeating yourself to a foreigner who isn't getting you.