1) In general, the production of IgM and IgG requires 1 wk and 2 wks, respectively. With one day old chick, the antibody is more likely from the dam, which can be confirmed by testing its mother's antibody.
2) To exclude primary infection in the chick, test its IgM by ELISA.
3) Get choanal or air sac swabs to test the presence of MG DNA by DNA PCR.