You can easily prepare a reliable homemade detector to test the presence of alcohol in breath by cheap and easily available chemicals and equipment.

Take Potassium Dichromate solution in a transparent plastic bottle. Add a few drops of sulphuric acid in the solution. Insert two straw pipes through two holes made on the cap of the bottle. Keep one straw inside and the other one outside the solution.

Now blow through the other end of the straw pipe (which is kept inside the solution)

If the color changes to GREEN from ORANGE, alcohol is confirmed. 

Chemistry behind this test-

Alcohol get easily oxidised to Carboxylic acid when oxidized by Acidified potassium dichromate solution. As a result the orange colour of potassium dichromate changes to green chromic ion.