T Cell Testing for COVID-19 Immunity

Research has repeatedly shown that infection and recovery from COVID confers "robust and durable" B cell and T cell immunity.

B cells produce antibodies that confer antibody protection and can easily be tested. That is done routinely.

What about testing for T cell immunity? Is that available? How do I do it? What will it show me?

T cell testing can help explain the common occurrence of 2 people being exposed to COVID and one producing antibodies while the other does not.