Life Total Dispute at FNM
The short answer is that there often ISN'T an easy way of resolving situations like this. If the players can recall the events of the game, and/or someone has been writing it down, then that may help you make a decision (like in the situation you describe). But otherwise you'll just have to take the information given and just decide what the life totals should be.
My advice then is more to do with how you deliver your decision, rather than how you come to it:
Make it clear to the players that you weren't playing the game, so you have to rely only on the information and stories you've been given.
Next, and you'll have to figure out how best to phrase it so it doesn't come across too bluntly - the players have messed up. One or other (or both) hasn't tracked the life totals correctly, hence the need for a judge. So if anyone is to blame, it's not you. You're here to fix the mess, you didn't cause it. (Again, phrase it nicer than I've written!).
Lastly, recommend pen and paper to them in future so that both players can track back the life totals. Hopefully it doesn't happen again :)