There might be a stamp on the glass identifying the window as low-e, but there is no universal requirement to mark windows that contain low-e characteristics although some organizations are trying to etsablish such a standard.
You could get the make, model and serial number of the window, contact the manufacturer or retailer, and find out if the window was made with low-e qualities.
I bought a manufactured home and paid a premium for low-e windows. I've looked everywhere I can see, but found nothing to identify the windows as low-e.
The home manufacturer says they tell low-e from regular windows by the little orange stickers on them, however the stickers are removed during the final cleanup prior to delivery. He referred me to the window manufacturer.
They told me to use the "lighter test." Hold a lighter flame close to the window glass. The first reflection should be very similar in appearance to the original flame. The second reflection should be noticably darker, almost purplish.
I called them back and told them that I had tried it in daylight and dark with no noticeable difference. They say they have an "instrument" that can demonstrate it, which they have promised to bring to my home. They added that they found the lighter test more reliable. If/when I get a better answer, I will post it here.
In the meantime, if anyone knows of a reliable way to test for low-e windows, I'd love to hear it.