テレビで試合見てたよ！って英語でなんて言うの？

All of these examples explains to the other person that you are watching a baseball game. It is usually assumed it is on TV unless you say you are streaming the game online.

"I'm watching the baseball game on TV." This explains to the person that you are watching TV and that you are watching the baseball game. Eg: What are you doing?- "I'm watching the baseball game on TV." "I'm watching the game on TV." This is another common way of explaining that you are watching tv and that it is a sports game. Eg: What are you doing?- "I'm watching the game on TV."

"I was watching baseball on the TV." "Was watching" Past continuous of watch. Or you could say, "I spent the evening watching baseball on the tv." Here you are telling someone what you were doing in the evening or how you spent your time. "I was watching baseball on the tv when you called." If someone calls you can tell them you were watching tv.

If you are at home, you can say: "I was just watching baseball." It is obviously being watched on your TV (or similar) so no need to mention the TV. If you use 'just' before a verb, it means 'very recently' (and can also mean 'only'). EG. "I've just been playing with the kids and am sweating so much!"

To say that you are watching baseball on TV, you can say "I am watching the baseball game on TV."

If you want to tell someone that you were just watching the baseball then you could simply say 'I was just watching baseball' by adding 'on TV' it explains what you were watching it on.