See attached -- this has the information. For most partners they will check it even if it is on 2 tickets:

https://pro.delta.com/content/deltap...e-policy-.html

Yet another example where DL is helpful while AA tells you to pound sand. My mother had an AA ticket from MIA-MAD and then a separate ticket from MAD onwards with Iberia (you know - one of the oneworld partners), AA refused to check bag through