Since Monero is anonymous, you won't see the origin address the funds you receive came from. If you want to know, for instance to credit a particular customer, you'll have to tell the sender to use a payment ID, which is an arbitrary optional tag which gets attached to a transaction. It's not possible to use standalone payment addresses, but you can generate an address that already includes a random payment ID (integrated addresss) using `integrated_address`:
This will generate a random payment ID, and give you the address that includes your own account and that payment ID. If you want to select a particular payment ID, you can do that too. Use:
Payments made to an integrated address generated from your account will go to your account, with that payment ID attached, so you can tell payments apart.
If you pay a merchant, and the merchant claims to not have received the funds, you may need to prove to a third party you did send the funds - or even to the merchant, if it is a honest mistake. Monero is private, so you can't just point to your transaction in the blockchain, as you can't tell who sent it, and who received it. However, by supplying the per-transaction private key to a party, that party can tell whether that transaction sent monero to that particular address. Note that storing these per-transaction keys is disabled by default, and you will have to enable it before sending, if you think you may need it: