How to find the drupal version of a site?

Hi there,

I have been charged with updating a Drupal site that has not seen so much as a patch in quite some time, however I have no idea which version it is... it's definitely on the 4 branch and thankfully it only seems to use core modules but I have no idea how to find out what version it made it up to.

I'm assuming upgrading it from 4.5.x or 4.6.x to 4.7.x should be fairly straightforward, is this correct or are the databases different? And if so will the following work?...

Take site offline
Turn of caching
Disable all modules
Back up files
Back up db
Delete files
Upload 4.7.x files
Restore all modules
Turn caching back on
Let everyone back in.

I'm also assuming that porting it, and it's custom theme, to 5.x would be significantly harder, is this the case?

Any advice on this upgrade would be appreciated... should I dump the contents of the cache table first for example?

Thanks,

Roger.