With the Conversation View in Gmail* that's not as much of a problem. Simply viewing the conversation will show you any replies or forwards you performed as well as any additional messages received in that conversation. But, understandably, some people would still like an indicator visible from the Inbox so they don't need to open the conversation to figure it out.
As a footnote, if the Conversation View is disabled, Gmail does add reply and forward markers to the individual messages. But the Conversation View is one of the main advantages of Gmail which means most people want it enabled.
While Gmail does not have such flags, it is possible to simulate them through the use of labels and filters.
To set this up, start by going to Settings->Labels and create two new labels: Reply and Forward. To save space, you may want to shorten the names to something like: RPL and FWD. It doesn't matter what you label them, just as long as it has meaning to you.
Next you will go to Settings->Filters and define two new filters:
Matches: from:(me) subject:(Re:)The search term "me" is used so that the filter will work for any custom from addresses you may have (Settings->Accounts->Send Mail As). Otherwise you'd need to search for each unique from address you might use. But note that from:me will not match a custom from address if "treat as alias" is not selected.
Do this: Apply label "Reply"
Matches: from:(me) subject:(Fwd:)
Do this: Apply label "Forward"
Basically the filter is looking for a message you sent (from you) that contains the standard Gmail tag in the subject for a reply (Re:) or a forward (Fwd:). It then applies the appropriate label. With these filters in effect, when you view conversation in the Inbox or any other label, they will have indicators if the conversation includes a reply and/or forward.
Of course since the labels are being applied to your outgoing message (not the original message) you need to be using the conversation view for it to make sense. The original message is not the one getting tagged (as happens with an e-mail client). And you will still need to view the conversation for the exact details: just what message of a larger conversation did you reply/forward, what you said, etc. But at least it will be easy to see which conversations have had reply/forward activity in them. And for most cases, that's enough for this method to be useful.
* Gmail is a trademark of Google, Inc. This page is not sponsored by or affiliated with Google.