I don't have a manometer to measure how much pressure of water my inlet valve is having, can I measure this by just having an empty container and have the valve fill it per x amount of seconds x amount of liters and calculate that to what my actual water pressure is?
In principle, yes, you can calculate the inlet pressure of a valve by measuring the mass flow rate as it discharges to atmospheric conditions. However, to so this, you need to know something about how the flow rate relates to the pressure drop across the valve, which will depend on the size and design of the valve. I actually work in the valve industry and a common concept that is used is the flow capacity, Cv, which is calculated using the following formula:
where $w$ is the mass flow rate, $N$ is a constant (depending on units used), $\Delta p$ is the pressure drop and $\rho$ is the fluid density. Essentially, Cv is a measure of how much mass of a given fluid will flow through the valve, for a given pressure drop - i.e. it characterizes the valve design as it relates to flow performance.
So, if you know what the Cv value is for a given valve, then you would be able to calculate the inlet pressure from the measured mass flow rate.