Is there any way to see whether a message forwarded to the telegram channel from another channel has been edited (and at what time)? 
Via API or some other tools.
Website version of telegram at web.telegram.org shows what time the forwarded message has been originally posted, but it doesn't display whether that message has been edited afterwards.
No, that's not possible. When you forward a channel post to any sort of chat on Telegram, the label "edited" will be removed.
Can I see whether message forwarded to the group has been edited and at what time?