As a home canner who processes well over 1,400 jars of food each year, I am always very appreciative of any free jars that come my way. Sometimes the jars come filled with food that I have to discard then clean the jars and other times the jars come empty with only cleaning needed. At any rate when my husband recently brought me home three cases (36 jars) of 500 ml jars, I was elated. After doing a bit of power canning, I am getting low on jars which is fine because that means the jars I have are filled but you know a home canner doesn't just can during the peak season of May to October. Oh no, a home canner tends to can year round filling jars as quickly as they are emptied.
Now, this is a clear example of what to look for in home canned foods. If the lid is bulging, buckled or otherwise not sealed then do not consume the product. Do not taste the product! Metal lids will bulge or when pressed in the centre will give indicating the seal has been compromised. Any product should be appropriately discarded. If you are using Tattler lids or glass inserts, the lids will not bulge but rather the lid and rubber gasket will pop right off. If that happens, do not consume the product. Chances are the food is only contaminated with bacteria BUT food born illnesses are easily avoided and there is no point taking chances. The rule of thumb with any food is if in doubt throw it out.