If you are using the old email format in Comcast, it does not have that option, unless you enable the account in a desktop email client like Outlook or Thunderbird. The new email format does have the feature. Two considerations are that a) Compliance on the part of the recipient is voluntary. They are free to ignore read requests. b) Not all email programs support the use of read receipts. So even if you send one, your recipient might not see it.
If you are on the new format, while in the compose page, click on "Options" to the right and above the message toolbar. The last item you see will be read receipts.