If you're using NinjaTrader 8's Alert Log, you'll see alerts appearing, sorted by time by default. If you have more than one chart that is showing the same symbol, for example, a 2 min and a 30 min, it may be difficult to tell which chart triggered the alert.
An easy way to identify exactly which chart triggered the alert is to double-click on the alert message. This will bring the chart that the alert occurred on to the foreground, and will flash the title bar of that chart a few times. This way, you can immediately identify the source chart of a specific alert.