The easiest way is to type the following command:

$ ps aux | grep 'apache'

You would get list of processes. I can see that the calling a /usr/sbin/apache2 is www-data. So the username of apache is www-data.

www-data  4126  0.0  0.3  32404 14296 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4132  0.0  0.1  26048  4520 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4134  0.0  0.3  32512 13964 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4135  0.0  0.1  26048  4516 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4145  0.0  0.3  31892 12964 ?        S    12:29   0:00 /usr/sbin/apache2 -k start