Pressure is defined as the force with which the liquid contained in a tank presses on a unit area of the wall. The air pressure on the outside of the tank presses inward on the sidewall. Inside the tank, the air pressure presses down on the liquid. These two air pressures cancel, if the air inside isn't pressurised. This leaves only the pressure from the liquid's weight to be calculated. The pressure is a function of the depth only and independent of container shape. The relevant equations are force=mass_g and pressure=force/area=mass_g/area=density_g_height, where g is the gravitational acceleration constant.
Determine the density of the fluid. This can be done by looking up the substance on the Internet or by weighing a sample of the liquid directly.
Measure the depth of the liquid at the point at which the pressure is to be determined.
Calculate pressure as density_g_depth, as derived above in the introduction. "g" is the gravitational acceleration constant, equal to 9.8 meters per second-squared or 32.0 feet per second-squared.
For example, at a depth of 2 feet below the surface of water (density=1.94 slugs/ft-cubed), the wall experiences a pressure of 1.94 slugs/ft^3 * 32.0 feet/s^2 * 2 feet = 124.2lb/feet-squared = 0.863 PSI (pounds per square inch).
The above calculations presume the density of the liquid does not vary with depth. Lowering a pressure gauge into the tank would allow a more accurate calculation, if such a variation were a possibility.