AQL is an admin and data browsing tool. It's not really Aerospike's SQL, as Aerospike doesn't natively implement a query language. Instead, all the Aerospike clients give you an API to make direct get, put, scan, query calls, and those are procedural, not declarative like SQL (where you state how you want the result and the server figures out a query plan). Piyush mentioned the predicate filtering API which is fantastic and lets you create complex queries over scans and secondary-index queries.
Specifically to your question about getting all the indexes, that's the type of thing you should use the info command for. Aerospike allows you to get and set config parameters through it, and get a wide range of metrics, run microbenchmark, etc. Everything you need for admin and monitoring.
You can run sindex
through the standalone asinfo tool, or you can call it using the info command that any client provides.
asinfo -v "sindex"