*Absence of a  proboscis definitely indicates that the insect is not a mosquito.*
 

Sometimes adult mosquito traps will have insects other than mosquitoes in them.  This page has a few of the more common insects that resemble mosquitoes, but are not.
 

The first thing to look at here is that this insect does not have a proboscis.  That is the easiest way to tell if an insect is not a mosquito.  Also, this insect has an enlarged thorax, or a large "chest" that mosquitoes do not have. 
 
 
 
 
 
 
 
 
 
 
 
 

This is a fly. Obviously this is not a mosquito.  The body is shaped nothing like a mosquito's.  It does not have a proboscis and has no scales on its wings.
 
 
 
 
 
 
 
 
 
 
 
 

This insect resembles a mosquito in body style.  However, there are no mosquitoes in South Dakota that are colored green like this insect.  Also, it has no proboscis and no scales on its wings.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Obviously, this is  not a mosquito.  The shape and color are not at all like that of a mosquito.  Also, there is no proboscis and the legs are not all attached at the thorax.