Darn - wrote up a big response and then something happened. Short version -
- self doubt, underestimation - check (triple check is probably more accurate there - we even had to test DS twice which showed we underestimated him even after testing the first time)
- early delays - check - DS was in free government speech therapy for a couple years as a toddler
- asynchronous - check
- behavior issues - check
Great advice above. Start with your school and figure out if they will do testing and if so when. Ours does but it is a group screening at the end of grade 3 which was too late for both of our kids. They needed stuff in grade 1 or things were quickly going downhill. It also wouldn't have been enough to unravel DS's 2e issues.
If they don't do testing (or it is much later, or you suspect something there might be more than *just* gifted) then consider outside testing. If you do this route it is good to know where your school stands on outside testing before jumping in. Do they accept outside testing? Are there certain tests they accept? timing (our board wants recent results), etc. Like suevv, testing changed EVERYTHING for us so in our case it was invaluable.
I will also say that I think you're in the right place. Gifted covers a pretty wide range many of which are not graduating high school at age 10. I think it would be a pretty safe bet that your DS is somewhere in that range - the challenge/fun is getting an idea of approximately where. For us getting an idea of just how far to the right they are has helped us understand just how unusual they are in a school system designed to meet the needs of most kids.