At our bank, we would follow these steps:
Have customer complete an affadavit of forgery - Make sure that items in question fall within your review timeline.
Do your own research - do signatures match or not? Check video, if available. Interview tellers.
If a true forgery - inform customer that you will refund funds and proceed by involving local police.
Determine if you have a suspect - sometimes a good video/DVR image will allow customer to help you identify who presented the forgeries - it always makes it more interesting if it is one of their family members.
If customer is adamant about bank refunding monies, make sure that they are aware that you will be filing charges with local PD and an arrest may be forthcoming.
Part of your interview may include asking where they think the suspect might have gotten a hold of the checks - if the customer was negligent in securing these items, they might share some responsibility. This is something that you might want to discuss with bank legal counsel.
My comments are absolutely no reflection of, nor influenced by, my employer - take them at your own risk.