How was Ainz able to perceive the threat of skeletal dragon beforehand and warn Nabe? In Overlord season 1 episode 9, Ainz asked Narberal to watch out above her and just like he said, a skeletal dragon appeared.

I don't think it was even explained in the light novel.

Can someone clarify how Ainz was able to perceive it?
If I remember correctly in the LN, Clementine talked about the existence of a Skeletal Dragon before it lands. Ainz then thought Nabe couldn't able to defeat it. That is why after some time Ainz says Naberal Gamma. This means she can get rid of the cover and use any spell she wants.