Short of sitting across the board with them and checking them for communication devices, you can't.
But just because someone makes good moves does not mean they are using a computer. Even mediocre players will make extraordinarily good moves -- sometimes they don't even know how good their moves are!
You can't!
Using another unit to know what will be the best next move...
But to realize that this is only a game to cheat oneselves (if they are... they surely cheating themselves). I just play an honest game (for fun), and if I win, I surely beat the cheater and the computer! 
I only use my own thoughts to play, check for my record for that. Great place place, great game, I wish everyone here used their own thoughts rather than outside assistance, I think most do. Let experience lead the way, meaning past mistakes and newly gained knowledge, but play an honest game. I wish you great success.
Rod
How can you avoid playing someone using a computer to make their moves ?