(Personally I do have a lot of sympathy with your thought that this is a plot hole - but here is a train of thought that might explain it)
To be fair on Ariadne, it is actually Cobb who first asserts that Saito is dead. From the script:
Cobb, I'm not going to let you lose
yourself in here! You have to get
back to your children!
Send Fischer, I have to stay
You can't stay here to be with her
I'm not. Saito is dead by now. That
means he's here. I have to stay
here and find him.
I know that making the distinction between Cobb and Ariadne first suggesting that Saito is dead seems very nitpicky, as we can ask the same question 'how does he know?'. However it does give a sliver of a reason why this might not be a plot hole. As pointed out earlier in the story when it is revealed how strong their sedation is:
So what happens if one of us dies?
That person doesn't wake up. Their
mind drops into Limbo.
Unconstructed dream space.
What's down there?
Raw, infinite subconscious. Nothing
there but what was left behind by
anyone on the team who's been trapped
there before. On this team... just
Cobb is the only person in the party who has been to Limbo and what was left behind by Cobb is the only thing filling Limbo. This residual Limbo can only have lived on within Cobb's mind - almost like Limbo is his dream. Cobb as the only visitor to Limbo has got some more ownership over it than the others until they inhabit it and claim it for their own. Ariadne and Cobb have only been in limbo a few minutes or perhaps a few hours, which would only be a very short time in the dream layer above, but perhaps Cobb through his stronger connection with Limbo and the world he created in it, he can sense that Saito has died and entered it with them.
Tenuous I know - but it is my only 'in universe' explanation.