You can get the supported nips of any relay with the following curl.

curl --header "Accept: application/nostr+json" 

For example the local relay i’m playing with.

[wolf@wolf-arch customrelay]$ curl --header "Accept: application/nostr+json" localhost:2700 |  jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   218  100   218    0     0   273k      0 --:--:-- --:--:-- --:--:--  212k
{
  "name": "Golang Relay!",
  "description": "relay powered by the relayer framework",
  "pubkey": "~",
  "contact": "~",
  "supported_nips": [
    9,
    11,
    12,
    15,
    16,
    20,
    33
  ],
  "software": "https://github.com/fiatjaf/relayer",
  "version": "~",
  "icon": ""
}

this functionality is documented in nip 11

https://github.com/nostr-protocol/nips/blob/master/11.md