What is the test for determining whether a clause in a contract is a penalty or a penalty?

In Dundas v. Schafer, the court also states at paragraph 55, that the question of whether a clause is a penalty depends on its construction and on the circumstances at the time of contracting. The court states that (1) the clause will be declared a penalty if the sum stipulated is "extravagant and unconscionable in amount when compared with the greatest loss that could conceivably be proved to have followed from the breach"; (2) if the contractual obligation is to pay a fixed sum of money by a fixed date and there is default, the obligation to pay a larger sum will be deemed a penalty; (3) there is presumption that a clause is penal when "a single lump sum is payable on the occurrence of one or more events some of which may occasion serious and others but trifling damage"; (4) "it is no obstacle to the sum stipulated being a genuine pre-estimate of damage that the consequences of breach are such as to make precise pre-estimation almost an impossibility."