The T-Detect test can tell you whether you had COVID months or even decades later. In recent history Spanish Flu T-cells were found to be present in elderly that survived that 1918 pandemic.
https://www.t-detect.com/"How does T-Detect COVID work?
T cells are the first responders of the adaptive immune system and activate the antibody response. While antibodies to SARS-CoV-2 (the virus that causes COVID-19) naturally wane and are detectable in the shorter term, T cell responses can persist in the blood long after antibody responses wane.1,2,4
The T-Detect COVID test can detect T cells in a blood sample that can remain for up to several months after symptoms appear.1,2,3"