Not sure but isn't our ult over a comm channel and the is is more ambient?
General rule of thumb to follow: if it sounds even-keeled and professional, it's a friendly ult.
Examples: Tactical visor activated, clearing the area, rocket barrage incoming, self-destruct sequence activated, the dragon becomes me.
If it sounds hostile, menacing, or is foreign, it's an enemy ult.
Examples: I've got you in my sight, die die die, Justice rains from above, nerf this, ryujin blah blah
Wait, you're right. Rein audio Ult is the same for both teams right?
Enemy Rein is hammer down, friendly Rein is just a metallic thud.
The hardest ults to distinguish between enemy and friendly are Reinhardt, Roadhog, Lucio, and Zenyatta. Both Rein and Hoggo say nothing during friendly ults (bonus points to Hog cause the enemy line is literally just really breathy laughter), and Zen and Lucio have really similar lines for both friendly/enemy ults AND they reverse the typical setup for Overwatch because the friendly lines are louder than the enemy ones.