Places you can check to see if you are an open relay:
http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=open+relay+test
Now, to your other question: You have done the correct thing with rcpthosts, but you also need to look at your /etc/tcp.smtp file (mine was really located at /etc/qmail/tcp.smtp)
See http://www.lifewithqmail.org/lwq.html#relaying for more info about how to set up this file, but it pretty much defines which computers are inside your network and you are OK with sending any mail for them-- and which ones are not.
Mine looks like this:
127.:allow,RELAYCLIENT=""
172.30.0.:allow,RELAYCLIENT=""
The first line alows localhost to send whatever mail they want. The second line allows anyone on our internal network to send any mail they want.
Sam
