You can see a Membership Level ID number in the bottom right of the screen for each Level if you hover your mouse over the Level Name for a level in the Setup > Levels section in WishList Member.
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You will see the Level ID number in the string right after #levels_access-
Example: https://wlmtest.com/wp-admin/admin.php?page=WishListMember&wl=setup/levels&level_id=1534276756#levels_access-1534276756
(In this example, the Level ID is 1534276756)
You should see that appear on the bottom left of the screen when you mouse over the Level name.