My reply from yesterday got nuked; apologies if it reappears and this is a duplicate.
Where can you find this? In the spec itself. If you actually read it, you will see in one of the first couple of paragraphs where it says Gr. 50 meets or exceeds the requirements of A 572. These specs aren't just number and letter designations; they're real documents with real requirements and real descriptions, and if you have AISC certification you're supposed to have copies of the relevant specs.
AASHTO M 270 is the same as ASTM A 709, maybe a year behind because AASHTO takes its updates from ASTM for this spec. There used to be a footnote in M 270 stating this explicitly, but it seems to have fallen off a few years ago. But up at the top of the AASHTO spec it also lists the equivalent ASTM spec. So M 270 lists right under the title which edition of A 709 it's equivalent to.
ASTM A 709 Gr. 50 is basically A 572 Gr. 50 plus Charpy requirements. A 709 and M 270 explicitly state that particular grades meet or exceed the requirements of A 36, A 572, A 588, A 992, etc.
D1.5 discusses the AASHTO & ASTM specs. D1.1 doesn't mention AASHTO specs at all; I guess they figured only the bridge people would ever be messing around with AASHTO specs. What kind of job are you working on that calls for D1.1 but AASHTO steel?
Hg