cat aliases

aliases shows information about currently configured aliases to indices including filter and routing infos.

GET /_cat/aliases?v

Might respond with:

alias  index filter routing.index routing.search
alias1 test1 -      -            -
alias2 test1 *      -            -
alias3 test1 -      1            1
alias4 test1 -      2            1,2

The output shows that alias2 has configured a filter, and specific routing configurations in alias3 and alias4.

If you only want to get information about a single alias, you can specify the alias in the URL, for example /_cat/aliases/alias1.