It is not always possible to know. I'll explain why, and how, when possible, to find the information you want.
First a bit of necessary background. When you "show original", you see the e-mail exactly as it was transmitted over the network. But what you don't see is the envelope. The envelope is like a paper envelope. It has a recipient address and a return address, and contains an e-mail. The envelope is discarded on receipt.
One way that you can receive e-mails for multiple e-mail addresses in Gmail is from a remailer, also called an autoforwarder. This covers a wide variety of circumstances. Mailing lists are one example: an e-mail addressed to the list is remailed to the list membership. Mail systems which let you give a forwarding order are another example.
These remailers take the e-mail out of the envelope and put it in a new envelope addressed to the forwarding address. The e-mail still reads as it did when it was sent. This is why, when you "show original", you don't see your Gmail mailbox in the recipient list. The sender addressed the e-mail to a different mailbox, so that's what you see.
You are in the same situation when you receive a blind courtesy copy (bcc) of an e-mail. The e-mail itself doesn't list the recipients of blind copies: that's what makes those copies a secret. This is why, when you "show original", your mailbox is not shown as a recipient.
E-mail often goes through several mail servers, like postal distribution centers, on its way to the recipient. Each one is a remailer. Each one discards the envelope, stamps the e-mail
Received, puts the e-mail in a new envelope, and hands it off to the next mail server. The stamp shows the date and time and other tracking information. Some (but not all) mail servers include the recipient address from the envelope that it discarded. You can view these stamps when you "show original". Look for the
Received: from transmitting server
by receiving server
id unique id
for → recipient address ←
; date and time
For more information, start with RFC 5321: Simple Mail Transfer Protocol, section 2.3, SMTP Terminology and section 4.4, Trace Information.