He had two solid reasons for doing this.
One – He was the ruler of the Swamps due to which he could actually see and sense every being there.
Due to that, he came to know that Aella wasn't in that region. Of course, that was why he was ready to leave. This was the main reason which let him conclude that Aella wasn't here.
Yes, even his sense of smell was helpful in smelling. When he tried to detect her scent, he couldn't get any.
Neither humans nor Aella, no one's scent was present there.
Second – He came to know that Balton was lying.
Since such a strong dragon was present here, trapped for long and attacking whoever approached it, no one in their sane minds would even dare to approach this place.
So why would someone like Balton enter this place?
Shin found it ridiculous but his anger for Balton increased. The next time they meet, Balton wouldn't come out alive.
Shin and the dragon soon entered the forest of Thuren.