Question

How to know where the pressute is leess in fluid mechanics?

Answer 1

Particle do NOT always move from high pressure to low pressure. They can move from a low pressure region to high pressure region and experience a decrease in speed.

The question is analogous to asking if objects always move in the direction of the net force on them. They don't, but they do always accelerate in the direction of the net force on them. If an object has a velocity in a direction opposite the net force on it, the object will slow down.

Consider a small parcel of fluid at a particular location. The pressure gradient at that location (plus the weight of the parcel) is the net force on a parcel of fluid there. This determines the direction the parcel accelerates, which could be different that its velocity.