If you are in a GUI terminal window, you are not in a login shell. And if you are in tty, you are sure in a login shell. To test these, you can use:

shopt -q login_shell && echo 'Login shell' || echo 'Not login shell'

or, simpler:

shopt | grep login

Example to use in an if statement:

login_shell=$(shopt | grep login | cut -f2)

if [ "$login_shell" = "on" ]; then 

    echo 'Login shell'

    # do stuff in login shell

else

    echo 'Not login shell'

    # do stuff in non login shell

fi