Switch to fullscreen, get width and height of the set resolution and divide them and check the ratio.

E.g. if it's set to 1680x1050 it's a ratio of 1,6 (16:10), whereas 1024x768 gives 1,333... (4:3) and e.g. 1600x900 is 1,77.... (16:9). Those three are the most common resolution ratios (with 16:9 being the least usual for the PC) so checking for those three should be enough.

And if your game is 3D you can use this FOV-calculator to see what your normal FOV calculates to in a widescreen resolution. Note that for 3D you don't necessarily need to support a widescreen resolution but rather the ratio. This is done in e.g. Armed Assualt which only allows 4:3 resolutions (like 1280x1024) and then let's you select your screens ratio. The game then just changes the FOV.

Note that I don't see widescreen resolution as necessary, but games look stretched in width if they are not fitted to use that resolutions, that's e.g. the cause for why I will never ever buy a commercial game that does not support WS (like the new Rainbow Six). But don't be afraid, I won't boycot your entry just cause of a lack for widescreens