Best way to check if transparent is really transparent

Hey guys

I made a box in 3D and rendered it out with a transparent background...

I import it in Photoshop, to finetune the image and then send it to my client as a transparent PNG so he can use it to compose it on his own backgrounds.

I THOUGHT this background was 100% transparent... (besides the obvious shadows).. But it turns out that the entire image has a very feint shadow cast on it. The client noticed this, when they put the image on their website (which is white). You could tell the image wasnt white but light grey.

I know why this happened (wrong lighting settings in my 3D programme)..but that is not my question.

My question is .. in photoshop, is there a way to visually see where there is 100% transparency and where there is still information?

These are the methods I thought of:

- alt click layer to see where the crawling ants turns up..but this feel like its very inaccurate. Would a single pixel 'show up'?

- export the image as an png, and re-import in Photoshop and place it in a bigger, white canvas. So you can see if the edges are different from white (this seem cumbersome)

Both of these methods dont seem very handy, so im wondering if theres a way to quickly and accurately see where there is 100% transparency and where not. Especially those very feint translucent parts are hard to discover for me so any help would be appreciated

ps Im on Latest photoshop Windows 64 bit