If it doesn't say in the regular video options you can always look at the resolutions, for example 640x480, 800x600, 1024x768 and 1280x1024 are 4:3 resolutions.
And another easy way is to look straight down or up and spin around and see if the ground, ceiling or sky makes weird wavy lines,
because then you will know that you have the wrong aspect ratio or resolutions.
However, if that doesnt work, (e.g. a game you can not look up or down in) then you only need to tilt your head to either side while you look at objects.
For further information on resolutions visit these pages:
http://upload.wikimedia.org/wikipedia/c ... dards4.svg