While there’s no meter or indicator to let you know, you’ll know when it happens just because it will level up and, if the conditions are met, it’ll evolve into the famous Jigglypuff.
You can do this by playing with it and washing it at picnics, as well as taking it for walks in Let’s Go mode. That’s because to level up Igglybuff, you need to raise its friendship levels to max. Once you have it on your team though you might notice that while you’re leveling it, it’s not really doing much to grow. You can find them pretty early in the game and they’re super cute so I can’t fault you. Well, let’s say you caught yourself an Igglybuff.
So, how do we go about it in Pokemon Scarlet and Pokemon Violet? The evolutionary line of Igglybuff, Jigglypuff, and Wigglytuff are pretty old in terms of when they were added to the game and yet, at no point do their evolutions involve hitting a certain level.