let’s start with some basic searching

no let’s do some google dorking

intitle:"api" site:"coinbase.com" #change this to any website you are targeting 

as we see we found the documentation

now let’s continue

inurl:"/api/v1" site:"microsoft.com" #change this to any website you are targeting
inurl:"/api/v2" site:"microsoft.com" #change this to any website you are targeting
inurl:"/api/v3" site:"microsoft.com" #change this to any website you are targeting

another one

intitle:"json" site:"developer.ebay.com" #change this to any website you are targeting

Searching GitHub

coinbase api 

coinbase api key