There are various tools and browser plugins that can be used to test your web site with different browsers. Each tool / plugin has its own quirks / limitations. The most reliable method for testing different version of IE is to use different virtual machine for each version. We used to be able to download the VM with IE6 from Microsoft. But recently Microsoft decided not to provide an image with IE6 anymore.
If we must still support IE6, and if we are lucky, we can find an old computer that has IE6 on it, or we can find a Windows installation CD that has IE6 on it.
If we have a physical machine with IE6 on it, and our developers can RDP into this box.
If we have a Windows installation CD that has IE6 on it, we can install it onto a virtual machine (using VMWare, VirtualBox, QEMU, or whatever), and our developers can RDP into this box. We will need to turn off automatic update. We probably need to disable copy-and-paste, and other file sharing mechanism. Considering that this box does not have automatic update on, and this operating system might no longer be supported by Microsoft, this box is not protected by antivirus, people that RDP into this box should not use it to browser other web site