What I usually use is Nico Nico Alert, which you can read about here.

Also if you join your favorite utaite's communities, when they're having a broadcast, if you visit their community page or go to http://live.nicovideo.jp/my (which you can access by going to Nico's namahousou home page and clicking on the top right hand menu for "My Page"), if you see a red button, it means they are currently broadcasting. Also you can use the currently airing namas and browse around. (I linked you the general namas, but probably you should look at the Yattemita section)

Also if you have a premium account and switch to Nico Zero, when someone from a community you have joined starts broadcasting and you're watching a video, a little notif will pop up.