Try the yaml language server by red hat, it comes with a docker compose validator.

But in general, off the top of my head, dashes = list. No dashes is a dictionary.

So this is a list:

thing:
    - 1
    - 2

And this is a dictionary:

dict:
    key1: value1
    key2: value2

And then when they can be combined into a list of dictionaries.

listofdicts:
    - key1dict1: value1dict1
    - key1dict2: value1dict2
      key2dict2: value2dict2

And then abother thing to note is that yaml wilL convert things into a string. So if you have ports 8080:80, this will be converted into a string, which is a clue that this is a string in a list, rather than a dictionary.