1830 Half Dime

Tough one - there isn't a single online resource for identifying Bust Half Dime varieties. The authoritative publication is Federal Half Dimes 1792-1837 by Logan and McCloskey; it's on the order of a $75 investment.

A really dedicated collector has many varieties listed here:

http://cbhalfdimes.tripod.com/id6.html

When confronted with trying to attribute a variety in an unfamiliar series, my first trip is always to Heritage Auctions. Their archive of completed auctions is possibly the single greatest online numismatic resource of all time, and just gets larger every day. Membership with access is free, if you're not already registered.

Search their Auction Archives by type, and then list the result by "Highest Grade First." The higher grades tend to have their variety described in the auction writeup - being higher-dollar coins, they get more attention - and can be invaluable in attributing the coin which you happen to be holding at the time.