How to know when/if I have too many indexes?
Your load profile is the most important factor in answering this question.  


If your load is read-intensive, you want indexes to satisfy your heaviest or most frequent queries.
If your load is write-intensive, index carefully. Index to satisfy a seek an UPDATE needs, for example, as well as your one or two most expensive SELECTs.
If your load is an OLAP one, index sparingly since you will be scanning the target tables anyway.


How do you know you have too many indexes?  


When you can see that some of them are not used by any queries.
A frequent DELETE, UPDATE or INSERT shows a query plan that involves several expensive index changes (i.e. a nonclustered index insert, update, or delete). Use your judgement to determine if the penalty on those DML statements is worth the gain you are getting from the indexes that have to be updated.