In Gmail, I think your only option is to click the arrow on the top right of the message, and then choose "Show Original".
The message is most likely to be sent in MIME format, so you can scroll down past the headers and look for something like this: Content-Type: text/plain
. In MIME format, there are unique strings (boundaries) between each message part, I believe that each email client chooses its own string to use. You can find out what is being used if you locate the following header:
Content-Type: multipart/alternative;
boundary="-----------=Sample_Msg_Part156165161321654"
In this case, the string -----------=Sample_Msg_Part156165161321654
is used to delimit the different message parts.
Here's an example...
Let's say that the message has the following content:
From: user@example.com
To: user@example.com
Subject: Test
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="-----------=Sample_Msg_Part156165161321654"
-----------=Sample_Msg_Part156165161321654
Content-Type: text/plain
This is a sample message. This is the text portion of
the message.
-----------=Sample_Msg_Part156165161321654
Content-Type: text/html
This is a sample message. This is the <b>html</b> portion
of the message.
-----------=Sample_Msg_Part156165161321654
... the plain text would look like this:
This is a sample message. This is the text portion of the message.
... while the HTML would look like this:
This is a sample message. This is the html portion
of the message.
Best Answer
As of October 2018 you can now make a change to your settings that will prevent this feature appearing on emails you receive. That change will apply to emails even if they aren't from Gmail users.
You can think of it a special filter that is applied to incommoding emails. If the application believes the smart_reply filter applies, then you will see the smart reply options when you read the email.
Because it is a special filter on incoming email, you have no ability to modify settings on the recipients Gmail account. If they allowed you that power it would be dangerous.