I need to get a check valve along the intake line to the pump for my pond. The purpose of the check valve is to retain water in the line when the pump is shut off and wouldn't lose prime.

From the manufacturer's manual:

"If after starting the pump you notice it takes more than three minutes for water to start flowing into the priming pot after being filled with water and energized, it is recommended to install a rubber flapper type swing check valve. The valve should be as low and as close to your source water as possible to maintain constant pressure and shorten prime time. The best location can even be underwater when possible as this can provide an air tight fit even without PVC adhesive. Some swing check valves have true union connection fittings that offer easier access for cleaning when needed. Never use a spring loaded check valve as it will reduce or stop flow."

I have a few questions.

(1) Should such a check valve be installed along a vertical pipe section or a horizontal pipe section? I read that most install a check valve along a horizontal section of the pipe but the manufacturer of the pump shows a vertical pipe with the check valve. Which is correct or does it not matter?

(2) If I install such a rubber flapper type swing check valve, there is a hinge where the rubber flapper swings. Is the elevation of the hinge important when installed? I mean should the hinge be higher than the flapper so it closes easier? Or does it not matter? I ask because I saw some check valves use clear plastic is it to see the position of the flapper or just to see if water is passing through?

(3) Are the clear plastic valves real PVC?

You always want to install a swing check valve vertically if possible.
If the check has to be installed horizontally the hinge point needs to be installed in the up position,meaning toward the sky
The clear check is usually the more quiet one and allows you to see if the check is in the closed position
As for material I’m unsure

If I install it in a horizontal position then either it needs to be a clear transparent valve to see the flapper, or if not transparent I need to inspect the valve first and mark a line indicating which side should be up? If I use a threaded connection check valve I then have to rotate the valve so that it's watertight but continue to rotate until the flapper hinge side is up. May be easier to use a valve with unions on both sides so the rotation is easier.

Yes or the if there is head pressure after the check I wouldn’t be too worried about seeing the rubber gate inside of the check

yead... the head pressure will work in your favor.

Quote
bsipps
if there is head pressure after the check I wouldn’t be too worried about seeing the rubber gate inside of the check

Backwards logic...you will only have head pressure in a horizontal check if it's installed with the hinge on top, if the hinge is on the bottom you won't have head pressure. So no head pressure only means that the valve is installed upside down, which you would have known had you been able to see the flapper and location of the hinge.

Quote

A Foot-Valve and Strainer is a non-return valve that is designed to be put on the end of a hose to suction lift out of tanks, rivers and lakes. The strainer stops any debris from finding its way into the pump and the non-return valve will then keep the prime in the hose as well making it easy to transfer from job to job.

[www.bing.com]

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

The check valve should have the words ( horizontal use up ) on the hinge side
Zoeller check valves definitely have those words on them