Well, you need to know the size/style of both your guitar and the case as a starting point. For example, is your guitar a dreadnought or a parlor? Because there really isn't complete standardization on the dimensions of what constitutes an dread or OM or parlor size guitar (one maker's OM may be significantly deeper than another maker's OM), you do need to get down to measuring the length, depth and width of the guitar to be certain. Especially if you know that yours runs towards an extreme. I think there is more standardization in sizing for dread size guitars than other sizes so a case that is advertised as fitting dreadnought guitars is a safer bet without exact measurements than one that is advertised for parlor size guitars which vary more widely.
When in doubt, measure and then call.