user29353: bitcoind json-rpc can do it, I think I saw this on blockchain.info but I don't see it now. 

user103097: getmempoolentry does it. However this is restricted to txes currently in mempool.

29353: <p>bitcoind json-rpc can do it, I think I saw this on blockchain.info but I don't see it now. </p>
 103097: <p><code>getmempoolentry</code> does it. However this is restricted to txes currently in mempool.</p>
