The rest is just filled in by logical deduction, or Occam's Razor. There is clear evidence that all birds have feathers, and no other animals alive today have feathers. There is now clear evidence that therapods had very similar skeletons to birds with feathers about 150 million years ago. There is also evidence that many therapods had feathers, a trait otherwise confined to birds.

Let our null hypothesis be that the origin of birds is unknown, and our alternative hypothesis be that birds evolved from dinosaurs. Given the similarities noted above (and, keep in mind, the evidence is much stronger for scientists who specialize in this area), do you think there is enough evidence to reject the null hypothesis and conclude that birds evolved from dinosaurs? If not, are there any plausible alternative hypotheses?

The great thing about science is that you never have to take someone else's word for it. Everyone has the right to be a skeptic, just keep an open mind and look at the evidence and you can draw your own conclusions.