It is good to contact other people already using the platform and ask them about their experience and a genuine forum is likely to have high-quality, relevant content that is regularly updated. If the content seems spammy or off-topic, it may not be a genuine forum. You can check the content by reading through some of the threads and looking at the quality of the posts.
You should also check the userbase, a genuine site is likely to have a diverse and active user base. If you see a lot of spam or inactive users, it may not be a genuine forum. You can check the user base by looking at the number of registered users.
You should also be on the lookout for any red flags, such as requests for personal information, links to unfamiliar websites, or requests for money. If you see any of these things, it is likely that the forum is not genuine