The way geo knight heating elements are designed/made they dont have cold spots. If it gets hot then you should be good.
Cold spots are more or less from having the element inside the heat platen spread to far apart, etc.
Here is what i would check:
Does it open and close smoothly?
Does the frame look straight? (for a clam shell press)
What is the cycle count on it? (Easy to check in the menu)
Is the platen in good condition...none to very minimal scratches?
Does it count down when closed? (if not its about $15 to fix, or could be a plug that came off the control board..2 min fix)
If its a clam shell (vs swing away) do the gas shocks hold it open? (if not it is about $15 each to replace and takes less then 5 mins to do all of them)
Does it beep when the timer reaches "0"? (If not it could be set to not use a beep in the menu or it is $15 for the part to replace it and takes about 1 min)
If the screen has a blue back light it is a newer model vs no back light and black text is an older one.
$500 for it sounds like a great price, even if you need to do a few things to make it basically new again.
Let me know if you have any questions about it. If you can post a few good pics and I can let you know what I think.