How do I test if an object has been instantiated?

[Stebo]

Dim F as Form
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.
.
Set F = frmSomeForm

If I do a quick watch on F prior to the set statement being executed, vb tells me the value of F is Nothing. However if I attempt to code:

If F = Nothing then
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.
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End If

I get an "Invalid use of object error message". How would I test if the set statement has been executed (i.e. F has been instantiated)?