Techniques are provided that relate to online advertising, such as in connection with an auction-based online advertising market, including techniques for determining evidence of prohibited use of user segment information by an advertiser. Segment information may be utilized in allowing a targeted advertising opportunity for an advertiser. The advertiser may, for example, have agreed not to use such segment information outside of the allowed opportunity. Particular users in a segment may be identified and utilized as indicator users for use in determining evidence of prohibited use of segment information by an advertiser in bidding occurring after the targeted advertising opportunity. For example, later advertiser bidding relating to near-threshold in-segment or out-of-segment indicator users may be analyzed. Alternatively or additionally, for example, bidding relating to false positive or false negative indicator users may be analyzed, where such indicator users may be falsely identified by an advertiser as being in-segment or out-of-segment.