The method that Lin Mu had used to detect the bloodline within Meihua's body was rather simple. But it was still not something that anyone could do. One needed to have a strong bloodline themselves and also have decent control over it to use this method.
What Lin Mu had done was to basically use the Great Slumber Bear Bloodline and let its aura trigger the hidden bloodline within Meihua.
The Bloodline within Meihua's body would take the foreign bloodline aura as a threat and would have no choice but to actively resist it. Of course, there was no real threat, as Lin Mu was fully in control of this aura. But the hidden bloodline did not know that.
It would simply consider that its owner was under threat and needed to be protected. And such effect would only happen when the bloodline could tell the foreign bloodline was indeed strong enough to threaten its existence.