Ontario's Residential Tenancy Act, s. 34 says:
The tenant is responsible for the repair of undue damage to the rental unit or residential complex caused by the wilful or negligent conduct of the tenant, another occupant of the rental unit or a person permitted in the residential complex by the tenant.
The burden is on the landlord to prove on a balance of probabilities that the tenant's wilful or negligent conduct caused undue damage.
For an example of this test in action, see CEL-04732-10 (Re), 2011 CanLII 27385 (ON LTB), where the Board found that the some of the damage was due to the tenant's negligence but that other damage was not proven to be due to the tenant's negligence. That is just one example that happened to be high on the search results. Other examples may be found in CanLii's database of Landlord and Tenant Board decisions.
Whether any particular damage is due to the tenant's wilful or negligent conduct is a question of fact that would be determined by the Board based on the evidence presented to it in a dispute.