There is no simple method.

Your best bets are:
1. Make sure your PC number is in your booking
2. When you book, put 'PC Platinum - Please Upgrade' in the comments box on the booking form
3. Look at priorityclub.com on the day you are arriving so you can see if the hotel still has plenty of rooms available or not for upgrades
4. Ask at the desk if you have been upgraded as a Plat, casually mentioning that you saw this morning that they had plenty of rooms available

The only other thing you can do is e-mail the hotel in advance of booking, say you are THINKING of staying with them and asking what they offer to PC Plats. If they say they will upgrade, print out the e-mail, take it with you and show it at reception.