I am asking about a method or a trick to measure (or guess) how acidic is food. Please don't suggest any expensive tool; this will be for home usage only.
Litmus paper is quite inexpensive. Edmund Scientific has 100 strips for US$1.95.
Some less precise methods using common ingredients:
Anthocyanin-containing foods (red cabbage, blueberries...) tend to turn blue in an alkaline, red in an acidic environment. Turmeric goes more lemon-yellow in acidic, more orange in very alkaline environments.
Baking soda can indicate strong acidity, it will activate and foam.
If there are green vegetables in a dish, their cooked colour gives you a hint about the cooking liquid (effect is not instant!)- if their color tends to go in the direction of olive drab, you are quite acidic; if they are unnaturally green your cooking liquid is rather alkaline.
Also, browning behaviour, especially of protein rich foods, gives you a hint - if it is hard to brown, your marinade might be just too acidic.