One way: have you ever delivered something which someone would pay $X,000 for? If so, then you are good enough to deliver something someone would pay $X,000 for.

I think many people get hung up on skill though. Clients pay for many, many things. Skill is only one of those things, and it doesn't dominate, either. Clients can also pay for:

  criticality of work
  urgency of work
  guaranteed availability
  hard-to-find expertise
  communication skills
  perceived attentiveness to needs
  lower perceived risk
  corporate politics
  measurable results
  warm fuzzy feeling

Of these, if you could only have one, I'd probably go with the ability to give clients the warm fuzzy feeling. You will never go hungry if clients get the warm fuzzy feeling from working with you.

Hat tip to tptacek, who taught me the importance of some of the above.