How to determine pH of food in order to avoid unsuitable storage containers
You can try wetting the tip of an ordinary pH paper, then put it into a sample of your food with that wet side. The capillary forces will suck up the liquid of your food and you should see a colour change in the formerly wetted area and above. 

PH paper is not a very reliable method, but it should give you a general idea and it is easily performed at home.