You could check for T-cells at some point -- if you had Covid, they are measurable even after many months. In contrast, IgG titers show up in only ~80% and decline steadily. Excellent recent paper:
> Here, we investigated SARS-CoV-2 antibody and T cell responses in matched samples of COVID-19 convalescent individuals up to 6 months after infection. Longitudinal analysis revealed decreasing and stable spike- and nucleocapsid-specific antibody responses, respectively. In contrast, functional T cell responses remained robust, and even increased, in both frequency and intensity.