I think you can guess that you're playing with a cheater if your opponent doesn't take time to think. Even in the most difficult situations, the time to respond is always short and approximately the same for all moves.

A real person must think, and, sometimes it requires more time.

I like to think. I play because I want to think, and it's fun. If I don't need to think to play, what's the point? Even if I'm a top-ranked player, I'll not earn money with that...

i cant accept the answer of fred
program players also taking a specific time (seconds) to arrange and get results from their tool.
 I am a fast player
i can play fast and win(this is by experience) i play like m drawing a pic or forcing opponent to place the coin in a specific place.
u can understand cheaters - 
1 . as fred said even difficult situations also they will take same seconds to play

2. normally u cant see any visible stategy.(computer is thinking 5 - 6 game ahead )

3. mostly 500+ players r programmers( some genius are there too)

thanks

If you really want to find s.o is cheater or not. You have to note the moves and analyse it with strong programmes like wzebra. The cheaters do the perfect moves at situations that no one , even world champions,can do it. 
But remember many high rated players know the opening books so dont judge them from their early moves

always makes me laugh when you easily beat an opponent and they ask for a rematch suddenly their playing style changes and you get creamed, that has happened quite a few times. I guess they think getting revenge by using an engine is justified.