How do I know what medications I can and can't take during pregnancy?

There are many resources so you are not totally on your own. Over the counter (OTC) drug makers are pressed to print 'Warning' &  'Cautions' regarding pregnancy and age groups that can and can't take their medication 'IN RECOMMENDED DOSE'. This is an important point. Regarding prescription medications (drugs) whomever writes for you to take the chemical, that healthcare provider needs to know you are pregnant or could be pregnant. Next the pharmacy that fills it needs to know, you are 'with child' -just in case the prescriber did not hear you when you said you are pregnant! That's the double check, but know that many medications have not been tested in pregnant humans. You can understand why!

So pregnant or not do you really need the medicine? Less is more sometimes.

There are unapproved marketed prescription drugs they have not been evaluated by FDA, and may not meet 'modern standards for safety, effectiveness, manufacturing quality, and labeling'. So watch out. Some healthcare providers think that unapproved drugs are "generic drugs." This is not true. 'Generic drugs are evaluated and approved by FDA, and are required, among other things, to be bioequivalent to a brand-name reference product'. (FDA)
The best internet drug information site is: www.fda.gov/Drugs. You will learn that studies show the average woman takes 3- to 5 medications while pregnant and more. Check out this site for pregnancy registry studies that track the risks from drugs taken during pregnancy or breastfeeding. The consumer portion of this national site has  the latest & updates drug information. Always worth exploring pregnant or not.