Currently there is not out of the box functionality on topic page level that would help indicating that the current topic is closed for replies. A visitor to the topic would only be able to tell by the deactivated reply buttons / missing quick reply. Only on the board level you can tell a topic has been closed by the locked icon (in the default styling) shown next to it.
It would be great if you could file a new product enhancement idea so that others can show their support for this improvement.
Looks like @miikka was quicker and just filed "Visual indication of a closed topic on Forum Topic Page" - Please share your support and add details on your scenario in the idea comments.
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