The question is a bit open ended, one possible way to approach this would be to take advantage of the ideal gas law and Dalton's law of partial pressure.
Butane (#C_4H_10#) from a lighter can be dispensed into an inverted eudiometer (like a big test tube with graduation markings) which has been filled with water and placed into a bucket containing water. The butane displaces the water in the tube. If the water level inside the inverted eudiometer is matched to the level of water in the bucket you can infer that the pressure of butane and water vapor inside the eudiometer equals the atmospheric pressure.
Using Dalton's Law of Partial Pressures and the vapor pressure allows for the partial pressure of butane to be calculated.
Now solve the ideal gas law for n.
n = PV/RT
You will need to have determined the mass change of the lighter to know the mass of butane in the eudiometer.
Now divide the mass of butane by the number of moles found using the ideal gas law. This could be paired with an empirical formula to determine the molecular formula.