Thanks. testmempoolaccept is the command I was looking for. According to https://bitcoincore.org/en/doc/0.20.0/rpc/rawtransactions/testmempoolaccept/ the result can be true or false.
I checked with a transaction that was already broadcasted and obviously it returned as false because the inputs are not present anymore.

{
    "txid": "c77c...042b",
    "allowed": false,
    "reject-reason": "missing-inputs"
}

If a transaction is not broadcasted yet, does this also check if the inputs are signed or not?
Or, what are all the "reject-reason"-s messages I can receive? Couldn't find it anywhere.