I had to file return by mail. how can I verify that it was received/accepted?

Unless you mailed it with a tracking number, you will need to wait about 4 weeks after mailing your paper return, you can check the status via the Where's my Refund site: https://www.irs.gov/refunds

After 2-3 weeks have passed you may also call the IRS customer service line at 1800-829-1954 to inquire regarding the status of your return.