How can you tell whether a chemical reaction favors products or reactants?

I'm not sure what you mean, but the actual way of determining that is by finding the constant of equilibrium of the reaction, when the conditions of the reaction are constant.

If the constant is 1, then neither products nor reactants are favoured. If it's larger than 1, then the products are favoured. When it's between 0 and 1, the reactants are favoured. 0 indicates that the reaction doesn't take place.

Now, different conditions can make favour either the reactants or products... so, it's difficult to tell sometimes, unless you have the figures or you perform an experiment.