How do you check that your page is compatible with every browser and version?

For example, if i want to know what my page looks like in:
- IE 5
- IE 5 Mac
- IE 6
- Netscape 4
- Netscape 6
- Netscape 7
- Opera 6

how would you go about doing it? Is there an online service available, or do you manually install every version possible?
Or do you wait until a user reports a bug?