How do you find out who manufactures Metoprolol ?

If you look at the label on your pills somewhere on it, usually under the drug name, it will tell you who made those pills.

There are many. See the link below.
http://www.accessdata.fda.gov/scripts/cder/drugsatfda/index.cfm - Type in metoprolol and click search. Then click on each drug name and it will take you to the manufacture details.