In Admin section and select Field Management, here you can export a list of all your API fields.
Regarding viewing the Opportunity tab of a lead, what CRM are you using? and, do you create Opportuinties on Account or Contact level?
Some companies still choose to create Opps on the account and not on the contact. When done on the contact and they have a contact role with that Opp, it's only then you will see that Opp tab populated with the Opp details.