Detection of Mycoplasma contamination in the cell culture should be performed with PCR.
You can design own primers specific for Mycoplasma or buy commercial PCR Mycoplasma Detection Kit(for example - https://www.abmgood.com/PCR-Mycoplasma-Detection-Kit-G238.html). Mycoplasma DNA in the cell culture supernatant is amplified via PCR and visualized using gel electrophoresis.