IRS "Where's My Amended Return" lookup tool:
https://www.irs.gov/filing/wheres-my-amended-return
The IRS says it can take 8-16 weeks (or longer) for the IRS to process an amended return. If you filed a paper amended return in the mail, the IRS status page says they are currently processing paper returns that were filed in March.
https://www.irs.gov/help/processing-status-for-tax-forms
