Terraform sdk usage, which out of Get, GetOk, GetOkExists with boolean?

Terraform Providers Plugin Development
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But how should we check if the boolean value is present in configuration?
To quote OP,
d.Get(“flag_presence”) gives false
d.GetOk(“flag_presence”) gives false and false

both d.Get and d.GetOk give same results both in case when value is present and the value if false and when value is absent from configuration. What is the way to distinguish these two cases?