Your best bet is to install Opera, NetScape and Mozilla onto your development computer and manually test your sites.
There are some third part applications that can browser check but the best are very, very expensive.
Some web authoring tools have minor browser checking built in. Dreamweaver can check its code against different browser types.
The Anybrowser site can help troubleshoot your pages: click here
Netmechanic is similarly useful: click here
Beyond that, you could do worse than get a computer magazine cover CD and install Mozilla, Opera and NetScape onto your own machine for testing.