The landlord should make an effort to contact the tenant (by writing or calling the tenant, for example) to determine if they have left the unit.

If the landlord believes that the tenant has abandoned the rental unit, then the landlord can apply to the LTB for an order ending the tenancy by filing an Application to Terminate a Tenancy and Evict a Tenant. This application is not mandatory, but if the landlord re-rents the unit without having the LTB confirm that it is abandoned, a tenant who hasn’t abandoned the unit could take legal action against the landlord.

Source: www.sjto.gov.on.ca