First, to monetize a YouTube channel, make sure you have an Adsense account or subscribe to platform advertising.
Then, to know the monthly income of a YouTube channel, you must first know its remuneration for 1000 views, this is the CPM (cost per thousand views). This varies greatly but generally, a youtuber earns between 0.50 and 0.60 cents for a thousand views.
Please note that CPM should not be confused with RPM (revenue per thousand views). The latter is usually lower, as it is calculated after YouTube profit sharing.
Otherwise social blade is an online tool which allows, by pasting the URL of the channel of your choice in the search bar, to be able to establish the amount of earnings that its owner receives.
Finally, if it is your YouTube channel, you will find this information in your YouTube Studio space.