You shouldn't have to... as I understand it, if it's showing up on your server, that means your server authenticated it. Given the general flakiness of all this software and Lemmy in particular, I wouldn't put too much reliability on that, but that's the theory.

If you do want to double-check it yourself, I know partially how to do it. You don't have to get the key from the database; it's probably simpler and safer to get it from your user's JSON. Here's a super-basic script to dump a fediverse endpoint's contents:

import requests
import json
import sys

def fetch_and_pretty_print(url, headers=None):
    # If headers are not provided, set default to fetch ActivityPub content
    if headers is None:
        headers = {
            'Accept': 'application/activity+json',
            'User-Agent': 'Fediverse dump tool via @mo_ztt@lemmy.world'
        }
    
    try:
        response = requests.get(url, headers=headers)
        response.raise_for_status()  # Raise an exception for HTTP errors

        # Try to parse JSON and pretty print it
        parsed_json = response.json()
        print(json.dumps(parsed_json, indent=4, sort_keys=True))
        
    except requests.RequestException as e:
        print(f"Error fetching the URL: {e}")
    except json.JSONDecodeError:
        print("Error decoding JSON.")

if __name__ == '__main__':
    fetch_and_pretty_print(sys.argv[1])

If I want to validate your comment, I would start by getting your public key via your user's endpoint on your home server. I could save that script up above as fetch, then run python fetch https://lemmy.mindoki.com/u/Loulou, and in among with a bunch of other stuff I would see:

    "publicKey": {
        "id": "https://lemmy.mindoki.com/u/Loulou#main-key",
        "owner": "https://lemmy.mindoki.com/u/Loulou",
        "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEArRwWZneP9efCrsymHDE2\nsJAHojjxE4A2Q3Hquwt7s/HPTAi3gKP7NKCRSH7XVPtGhieJdtDeoLMkitvZXCUX\nS1pZArTYihuLeOwbB+JrAHZpWr1sYpazspUPvl3MhDAOOCCAnSeqsMNPNd8QX1Tf\nN/3Bp4PRVmp9E968L61h93L5N3B7VxZ37kbzKFXrhmU6qFQbAoVQvHtojCD6WqR2\nMb84eJy5QBN+0SjvGR8LRE0iJZiwYvVXKNoEyOqr4Fw8YnELi3TYbfxX++0uXw97\ne+/rFgaa/QVCSopUbHkuX/ZfjzCdBAI+aqXsbmYLgdxdRDHur0k53aCh3u0t/IDL\nHQIDAQAB\n-----END PUBLIC KEY-----\n"
    },

I don't know off the top of my head how you could navigate your way to the fediverse JSON for your comment, or how to verify its signature once you find it (I tried to get the post by dumping your user's outbox and the lemmy_support community's outbox, but neither of those worked the way I expected it to), but that all might be a helpful starting point. I know that according to the docs, anything that was created by your user and then federated is supposed to be signed with that key so that other servers can authenticate it.