I usually just try to give them a detailed picture of my symptoms, as far as I can, and listen to them. When it seems like they are getting ready to wrap up, I simply bring up my concerns, if they have not been addressed. The attitude should be one of seeking their reassurance that they have adequately considered more serous possibilities.

It does not always work, but a lot of doctors in my experience take some time to kindly explain why they think the other possibilities I'm concerned about are unlikely, and sometimes my questions actually prompt them to add some additional investigations.