How do Trainers keep track of which Pokéball belongs to which Pokémon?

There are multiple explanations to this situation:

In Universe

1. Carry Restrictions
   There is a common theme across all Pokemon mediums (Anime, Video Game, Manga etc) that a trainer can only have 6 Pokemon available to them at any given time. This would make it easier to remember which is which. Especially given:
   • A lot of trainers only carry a couple of Pokemon. For example, when we first meet Brock, he only has Geodude and Onix. He later catches/is given Zubat and Vulpix respectively, leaving him with 4 out of 6.
   • Pokéballs aren't just all carried in a loose Pocket, They're stored like shotgun shells along the belt, or in a specialised backpack pocket or container. The Anime shows this best, with Ash's Pokéballs lining his belt. Sure, it'd take a bit of memorisation, but assuming he always puts his Pokéballs back in the same place, he would know which one's which:
     
2. Ball types
   There are many types of Pokéballs with different designs. They aren't always shown in the anime (see Stock Footage below), but they are heavily used in the games to alter catch chances (or even just for aesthetics).
   • Different Ball colourings could therefore be used to distinguish between Pokemon.
3. Engravings & Symbols Sometimes, Pokeballs are shown having special markings or symbols denoting the Pokemon inside. (Note that these sometimes may be accidentally (or deliberately) left off for production reasons, see Stock Footage below).
   • This scene is in the very first episode, and the Pokeballs say 'Charmander', 'Bulbasaur' and 'Squirtle' respectively:
     
   • In the same episode, we are shown Pikachu's Pokeball, which has a little Electric symbol painted or engraved on:

Out of Universe

1. Off-camera
   It's boring to watch the protagonists try and figure out which Pokemon/Pokéball is whose, so these 'scenes' are always skipped over so we can get back to the exciting stuff of the episode.
   • You may note the same thing happens with cooking/setting up camp, and shows/festivals. One second the protagonists are just walking along the road, and in the next scene they have had a picnic and everyone's had their fill. Or they enter town and a festival is happening, and suddenly they're all dressed for the occasion.
2. Stock Footage
   It's easier to use 'stock' shots of Pokéballs, especially those being thrown.
   • In scenes with a large amount of Pokeballs, it would be easier to just copy the 'standard' Pokeball template a hundred times.
   • Writing engravings or altering colours for each scene is hard and time consuming. Instead of customising them each scene for each Pokemon, a standard ball gets the job done and doesn't detract from the current episode/story.