Am I obligated to pay for the drinks I consumed at cafes, knowing that a friend claimed to have paid the bill, and it is possible he did not?

If a person invites you to a restaurant and pays the bill, there is no harm upon you. However, if each person orders for themselves, and then one of you tells the restaurant owner, "The price is on me," this is a guarantee, and your financial obligations are not discharged except by payment. Similarly, if one of you promises to pay for what you have ordered for yourselves without informing the restaurant owner, and then does not pay on your behalf, your financial obligation remains active with the debt, and you are required to settle it.