Identifying current version

Hi guys.

I've just been assigned to move a client's support site to a new server and I'm new to this software.

The support site is not working on the new server. The hosting support says that the current version is not set up for PHP 5.5, but PHP 5.2 (which is the old server) and they are recommending that we upgrade the support software.

Can you tell me how I find out which version is currently set up on the site?

Any help would be appreciated.

Thanks.