Your doctor will only ask you to jerk off in front of him (or her) for a few minutes in order to see if you can get a hard-on or not. If you're still limp after a few minutes (and he/she didn't have to tell you to alter your technique), then you'll likely be given a prescription for either Viagra, Cialis or Levitra. If he/she gives you the option, go for the Cialis.
p.s. There are times when your doctor might need to measure your hard-on with a measuring tape. Don't worry, they'll wear latex (or non-latex) gloves when doing so.