Bitcoin RPC: Can I test for standardness? I want to test if a script or is standard or not. Is it possible with Bitcoin Core's RPC?
The top-level type parameter of the decodescript RPC can tell you if a scriptPubKey is standard.  For example:

bitcoin-cli decodescript deadbeef
{ 
  "asm": "OP_UNKNOWN OP_CHECKSIGVERIFY OP_UNKNOWN OP_UNKNOWN",
  "type": "nonstandard",
  "p2sh": "3PbdV4zdrjtFD4fyYebGR4oxeatrSzb2d1",
  "segwit": {
    "asm": "0 5f78c33274e43fa9de5659265c1d917e25c03722dcb0b8d27db8d5feaa813953",
    "hex": "00205f78c33274e43fa9de5659265c1d917e25c03722dcb0b8d27db8d5feaa813953",
    "reqSigs": 1,
    "type": "witness_v0_scripthash",
    "addresses": [
      "bc1qtauvxvn5usl6nhjktyn9c8v30cjuqdezmjct35nahr2la25p89fssxejht"
    ],
    "p2sh-segwit": "37CpdZEgXUB7LS7rvTLsYopRifDf76eZrZ"
  }
}


However, I don't know of a way to test whether a P2SH/segwit redeem script is standard using the RPC.  If, instead, you want to know whether or not a particular spend would be accepted to your node's memory pool, Bitcoin Core's development branch (master) features a new testmempoolaccept RPC; here's it's current help:

testmempoolaccept ["rawtxs"] ( allowhighfees )

Returns if raw transaction (serialized, hex-encoded) would be accepted by mempool.

This checks if the transaction violates the consensus or policy rules.

See sendrawtransaction call.

Arguments:
1. ["rawtxs"]       (array, required) An array of hex strings of raw transactions.
                                        Length must be one for now.                                                                                                           2. allowhighfees    (boolean, optional, default=false) Allow high fees

Result:
[                   (array) The result of the mempool acceptance test for each raw transaction in the input array.
                            Length is exactly one for now.
 {
  "txid"           (string) The transaction hash in hex
  "allowed"        (boolean) If the mempool allows this tx to be inserted
  "reject-reason"  (string) Rejection string (only present when 'allowed' is false)
 }
]

Examples:

Create a transaction
&gt; bitcoin-cli createrawtransaction "[{\"txid\" : \"mytxid\",\"vout\":0}]" "{\"myaddress\":0.01}"
Sign the transaction, and get back the hex
&gt; bitcoin-cli signrawtransaction "myhex"

Test acceptance of the transaction (signed hex)
&gt; bitcoin-cli testmempoolaccept "signedhex"


You could also test out the complete script using regtest.  By default regtest has the standardness checks disabled, by you can re-enable them by starting bitcoind with the -acceptnonstdtxn=0 parameter.