How can you tell check valve....?

[Neptune 555]

I have purchased and installed the battery powered Air supply battery pack - that automatically goes on when powers goes off. My question is how can you be sure you have installed your check valve the correct way? I hesitated to install this solution b/c I can't tell if my check valve is right side up or upside down? A huge fear is a reverse tank drain b/c have this wrong. Both systems work when power is out... but how can I be sure a reverse siphon is not going to happen?

 

[nautical_nathaniel]

Blow on it and if air goes through then you have the right direction. A good check valve will always have a direction arrow on it as well.

 

[mfinn]

but how can I be sure a reverse siphon is not going to happen?

By placing the return in the display tank high enough so the sump can handle the back siphon amount when the the power goes out.
Many rely on a hole drilled in the return pipe just under the waters surface that acts as a siphon break. But that isn't always dependable.
A snail could decide to crawl over the hole at the wrong moment or algae can plug the hole.