You find the limiting reagent by calculating and comparing the amount of product each reactant will produce.
The usual steps are
1. Balance the chemical equation for the chemical reaction.
2. Convert the mass of each reactant into moles.
3. Use stoichiometry for each reactant to find the mass of product produced.
4. The reactant that produces the least amount of product is the limiting reagent.
Let’s look at respiration, one of the most common chemical reactions on earth.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
What mass of carbon dioxide forms in the reaction of 25.0 g of glucose with 40.0 g of oxygen?
Step 1: Determine the balanced chemical equation for the chemical reaction.
Step 2: Convert all masses into moles.
25.0 g C₆H₁₂O₆ ×
40.0 g O₂ ×
Step 3: Calculate the mass of product from each reactant.
0.139 mol C₆H₁₂O₆ ×
1.25 mol O₂ ×
Since glucose gives the smaller amount of product, it is the limiting reactant.
Here is a video which discusses how to determine limiting reactants.
video from: Noel Pauller
Well organized, example and nice pictures here: http://chemwiki.ucdavis.edu/Analytical_Chemistry/Chemical_Reactions/Limiting_Reagents