IF they know you're an illusionist, and IF they have reason to believe the Wall of Force isn't there...
This typically involves a Spellcraft check, or detect magic, or True Seeing. Therefore, it's only a problem if you're facing casters, in which case you should have already cast an invisible Obscuring Mist.
I think you are thinking of Silent Image - which is indeed a spectacular spell.
Shadow spells aren't figments - they are quasi-real, and always provide a saving throw - even if the spell they are emulating doesn't.
Also note that Shadowcraft Mages can increase the "reality" % - but not until mid levels. Furthermore, you need a DM that allows the heightened spell trick (which not all do) if you hope to get 100% reality (and even then there is still a saving throw)
Shadow spells are great spells - but generally they are not as good as the spell they are pretending to be (their strength is in versatility)