What it is and where it comes from
Citric acid is the acid in lemons. For food it is made by a mould (Aspergillus niger) that ferments sugar or molasses. Its salts, the citrates, are made from it.
Where you find it
Soft drinks, sweets, jams, tinned food, sauces and almost every sour product.
What to look for on the label
Nothing to check for halal.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.