Some DSP´s show the number of billable impressions. In DBM, for example, “the number of impressions that reached the intended or guaranteed audience. In a campaign with an audience guarantee, impressions that don’t reach the agreed upon audience (as defined by Nielsen, comScore, vCE, or other 3rd party) won’t be considered billable. Depending on inventory type, partial impressions may be reported.”
Also if the DSP detects ad fraud on the impressions, it can not include them in the billable impressions. See here how Doubleclick reach an agreement with several exchanges to refund invalid impressions.