RE: Is there way to find that a record was created using Web api in Dynamics 365
Sadly not - unless the records created via the web api requests are created using a different user account.
It's probably worth adding that when we start looking at the unified interface you will see the it is really a Javascript framework which itself uses the webapi to retrieve, create and update all the information that is displayed.
One solution would be to create a hidden field within the UI and add a piece of javascript (which will only run on the client) to put a value in that field when the record is first saved - but this doesn't exist out of the box so you wouldn't be able to identify where the records created prior to the javascript being implemented were created.