how can i tell if a user isn't a member on here anymore

If their account has been deleted then you'll see an "X" in their picture instead of the normal picture or security fuzzed picture. Other than that, there's no way I know of to figure out if someone is still logging in or not and is just lurking.

This notice will be in place of the former user page

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www.similarworlds.com/username

If they just changed their un, their new profile will come up, but if they're gone it'll tell you