A large body of interruptibility research has attempted to minimize disruptions caused by smartphone notifications. Yet, little research has explored ways to enable users to selectively attend to notifications, which can occur as early as users first notice the notification alert and start to speculate about its source. Nevertheless, users' speculation may not be always accurate. We took the first step in helping users make speculations about notifications to facilitate selective attendance. We developed Notiware, an Android app that helps users speculate about notifications by generating alert assistance when it detects that the app of an arriving notification cannot be correctly speculated. An ESM study with 30 users who used Notiware for 4 weeks shows that, overall, Notiware increased the accuracy of participants' speculation of notification app by 28%. Moreover, Notiware helped the participants skipped irrelevant notifications 1.21 times more often than without the assistance.