The simple answer is there is never enough research. Just when you come to a conclusion, you find out your conclusion was wrong. Some of the members here call up museums and ask to inspect originals, you can learn things from auction sites which sell things from the period, you can look in old books (Google books is invaluable for this, as is Archives.com and Guttenberg) as well as knowing who has specialized in such research and talking to them. They'll often have a bibliography from which you can base your own research, as well as giving you a boost.
As far as having evidence enough for an opinion, you'll have to realize there's never enough info, but you can say "My research to date suggests..." and it allows you to increase your knowledge without stating an absolute.
Visitors can be funny people. Sometimes they want to pull information out of you and sometimes they'd rather you said nothing while they stand and stare at your funny clothes. They're often interested in what you are DOING rather than what you know, so explaining what you are doing is a good way to present your information. If you're in a store, ask them about things they are familiar with, brands they might buy that were in use during the time, prices on things they might be familiar with, check out procedures... a host of places to start from there. Show them some of the things you sell, tell a story about them.... Those braces for instance. (there were different styles)
Questions are a good thing. Doing your own research is fantastic and a huge step in the right direction, but never feel ashamed to ask questions.