What it is and where it comes from
Isinglass is a form of collagen taken from the swim bladders of fish. It is used to clear beer and wine of cloudiness, and settles out with the yeast. As a fish product it is halal. The drinks it is used in are a separate question.
Where you find it
Mainly beer, cider and wine. It is rarely used in food.
What to look for on the label
It is a processing aid, so it is usually not on the label.
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.