We used to say that you should look for evidence that they comply with the ISO 12312-2 international safety standard for filters for direct viewing of the Sun. But now the marketplace is being flooded by counterfeit eclipse glasses that are labeled as if they're ISO-compliant when in fact they are not.
https://eclipse.aas.org/eye-safety/iso-certification
https://eclipse.aas.org/eye-safety/iso-certification