How can one tell if ignition switch needs replacing?

Could some one let me know when the ignition switch needs replacing?
What are the tell tale signs?

Any help is appreciated.

Thanks
Davey Carpet

Its been my experience that if the key won't turn the ignition (assuming you're using the correct key) or if the key gets stuck in the ignition or won't turn to ACC, you will need to replace the ignition switch.