How to catch the fees on regtest mode? I'm developing on regtest mode and it seems that the fees I specify in every transaction are not included on every new mined block (generate). I only view (listunspent) a new 50 btc coinbase utxo at the miner node. So how can I catch the fees on regtest mode?
I guess you are missing the coinbase immaturity here.
Coinbase transaction can only be spent after 100 blocks.
A single generate will not make the coinbase-transaction appear in listunspent. If you do a generate 101 it will (because you mine 100 blocks on top of your "fee block").

Then you should get something like:

{
    "txid": "11cc285148b32fd312260a67193875420931b8511b5f34f2991feb5d94901b8e",
    "vout": 0,
    "address": "n3y4wCJcY8zzim3CWuKpFi6BPPAf6DzhVm",
    "scriptPubKey": "2103559b1d5cd126e03b8ee0887ffc1ffd74d34fae49e2e2e65f7708071b032aabf5ac",
    "amount": 25.01920001,
    "confirmations": 101,
    "spendable": true,
    "solvable": true
  },