How to know when a carbide insert is dull???

[Ageless]

This might seem like a dumb question and it probably is but I do not know how to gauge whether
a carbide insert used in a milling machine needs replacing.
I understand that if the insert is chipped it needs to be replaced, I understand how to evaluate HSS
end mills for sharpness. Is the answer in how wide the corner point becomes with use?
I appreciate any information I might receive. :bow:

 

[websterz]

Run a thumbnail down the edge, from an unused point to the one in question. If you feel any difference it's time to rotate to a new edge.