If you have 8 cars and 48 tires, which one is limiting? Which one will you run out of first?
In this example it is easy to "see" which is the limiting and which is excess. When looking at quantities of chemicals it is not as easy to "see."
To determine which chemical is limiting, convert from the given reactants to a product (it does not matter which). Whichever reactant produces the least amount of product is limiting.
4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)
If you have 2.00 grams of ammonia and 4.00 grams of oxygen gas which is limiting?
Because the oxygen produced less of the product nitrogen monoxide, oxygen is limiting.