My son just got a Breakthrough Elite Trainer Box. He's got several basic and stage 1 Pokémon and I'd like to find out what Pokémon those cards evolve to. What's the best way to find out how basic and stage 1 Pokémon can evolve?
TL;DR - Simple way
Use official Pokémon TCG Card Database where you can find cards that evolves from a card you have.
Explanation and hard way
Pokémon TCG uses the same evolution system as video games and anime, i.e. every Pokémon has determined number of possible evolutions. However, most of them has only one or zero evolutions. Bulbapedia List of Pokémon by evolution families has all possible evolutions available, if you don't know them at all. Mega evolutions are included here.
TCG also has its own additions to that system, although they are rather simple.
Sun & Moon evolution mechanics:
- any Pokémon other than ♢ Pokémon
- normal evolution rules: Basic → Stage 1 → Stage 2
- some Baby Pokémon are excluded, i.e. Pikachu is Basic Pokémon
- Pokémon ♢ (Prism Star)
- card is Basic even if the particular Pokémon is normally not Basic
Deprecated evolution mechanics:
- Pokémon EX, Pokémon SP, and Pokémon ☆ - always Basic, can have Lv.X or Mega Evolution
- Pokémon BREAK and Pokémon Lv.X - power-up that enhances some Pokémon
- Mega Evolution Pokémon
- evolve like Stage 1 (are put onto Basic Pokémon)
- player's turn ends immediately after Mega Evolving a Pokémon
- Restored Pokémon - evolve like Basic Pokémon (can evolve into Stage 1 Pokémon)
- Baby Pokémon
- generation 2 and 3: evolve into Basic Pokémon
- generation 4: Baby Pokémon are Basic cards that don't evolve at all
- all other mechanics follow normal evolution rules
To get all cards your Pokémon can evolve into, find if it has an evolution and look it up in Pokémon TCG Card Database, which includes released cards, or in Bulbapedia List of Pokémon by evolution families (select a Pokémon, then select In the TCG), which includes also very old cards and cards yet to be released in English.