How to determine pH of food in order to avoid unsuitable storage containers In an acidic (lemon-containing) sauce I had placed a metallic spoon. A few days later it had a metallic taste. 

Stainless steal might be a significant source of nickel, chromium and iron [1]. Also, using aluminum utensils and packaging when cooking or storing food can contribute to the daily aluminum intake [2].

I want to avoid storing food in metallic containers which could increase my intake of metals. 

The pH of some foods can be found here, but in some cases it's not of much use (when the food is a varying mixture of ingredients). 

Is there an easy way to determine the pH of food? (whether the food is colored like a tomato sauce or transparent)
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.