Since you have a Linux environment, you can run this on your console:

locate bin/php

And then for anything that looks like a PHP binary, get the version. The output for me for the above is:

/home/xx/Development/Personal/Project1/webapp/bin/phpunit
/home/xx/Development/Personal/Project1/webapp-backup/vendor/bin/phpunit
/home/xx/Development/Personal/Project2/app/vendor/bin/phpunit
/home/xx/php-threaded/bin/php
/home/xx/php-threaded/bin/php-cgi
/home/xx/php-threaded/bin/php-config
/home/xx/php-threaded/bin/phpize
/usr/bin/php
/usr/bin/php5
/usr/local/bin/php-cgi
/usr/local/bin/php-config
/usr/local/bin/php53
/usr/local/bin/phpize
/usr/sbin/php5dismod
/usr/sbin/php5enmod
/usr/sbin/php5query

Out of those, there are a few that look like PHP binaries. So let's get the version for each:

/home/xx/php-threaded/bin/php -v
/usr/bin/php -v
/usr/bin/php5 -v
/usr/local/bin/php53 -v

That will give you the versions of PHP you have installed.

I wouldn't bother deleting an old version, it might remove files that will stop things working. You can just configure the console version, or the Apache version, to use the version you want.

In answer to your supplementary question: it seems that you've followed the instructions here to add an unofficial repo to your version of Ubuntu, since the standard repo does not support 5.5.

We discovered together that the way to get it working was first to upgrade Apache from 2.2 to 2.4:

sudo apt-get upgrade apache2

It should be noted that this can cause some vhost repair to be required, as some Apache directives changed in this version. Once you have done that, you can get the new version of mod_php:

sudo apt-get install libapache2-mod-php5

You can make an index.php file with

<?php phpinfo() ?>

I got it to work with the following commands:

# Full version
php -v | head -n 1 | cut -d " " -f 2

# Major.Minor version
php -v | head -n 1 | cut -d " " -f 2 | cut -f1-2 -d"."

Use dpkg to find the installed package versions.

dpkg -l | grep '\(php\|mysql\)'