My mom said that she's type o blood type.i'm type ab.so either she was wrong, i'm adopted (not possible :-) ) or i have cis-ab. how would i find out?

Genetic testing: As you said, first thing should be to test the blood groups on the two of your. You may not be adopted, but you could have been switched at birth with another baby. If you must both of you and preferably you and both of your parents could get genetic testing to sort out the matter, especially cis AB. Blood group B can also be acquired in some disorders, eg colon cancer. You may consult this paper:https://www.ncbi.nlm.nih.gov/pubmed/61608