How to differentiate inbound and outbound webhook?

You can define the call direction rely on an event with the extension id in the parties object. See this example.

// No extensionId => internal call
"parties": [
        {
          "accountId": "809646016",
          "id": "p-a0d7bca1f4a83z18613348f51z3343dxxx",
          "direction": "Outbound",
          "to": {
...
// has extensionId => call to/from this extension
"parties": [
        {
          "accountId": "809646XXX",
          "extensionId": "62242421XXX",
          "id": "p-a0d7bca1f4a83z18613348f51z3343dxxx",
          "direction": "Inbound",
          "to": {

You can read this blog for more explanation about telephony session events.