How to make sure the fluid goes in the direction it suppose to go?

 For short, the answer lies in a concept widely used in pipe engineering: hydraulic load or just load. Unfortunately, this concept is not well understood. 

The hydraulic load can be understood as the amount of energy available to drive a fluid through a pipe or conduit. And if you compare the load at two different points along a pipe you could easily check for the direction in which the fluid travels.

The hydraulic load can be, mathematically, written as:

$H_L=\dfrac{p}{\gamma}+z$        Eq. (01)

where:

$H_L$ stands for the  hydraulic pressure at a given point along th pipe,

$p$ for pressure at that point along the pipe,

$\gamma$ for specific weight of the fluid,

$z$ vertical height of that point along the pipe.

As you may  have already noticed: the hydraulic load only makes sense when talking about a given location along a certain pipe.

Fig. 01 Flow direction cases according to the hydraulic load $H_L$ at two different locations.

In case 1 in Fig. 01 the flow travels upward and the only way of making sure this is so is by checking that $H_{L1}$ is greater than $H_{L2}$. Otherwise, the fluid will travel backwards.

 This concept is a mechanical balance coming from the Bernoulli equation. However, estimations on how greater $H_{L1}$ is with respect to $H_{L2}$ depends on some pipe features and equipments taken into account in the general energy balance equation.

Any question? Write in the comments and I shall try to help.

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Ildebrando.