The Dyson V6 has 2 filters a pre and post motor filter. The Filter on the back has a Max button to change power modes, this is how to tell its a Dyson V6 Vacuum, also the quick release button on the front is grey.
The Dyson V7 only has the one filter on the top and no quick release button as the buttons are now on the attachements.
The Dyson V10 only has the one filter . this is situated at the back of the machine and is a solid purple colour (not opaque),
also has a large forward sliding bin rather than the older bottom empty machines.
The Dyson V11 is very similar looking to the V10 but the easiest way to tell the difference is the filter is opaque purple.