Originally Posted by SystemicAnomaly
I use a variation of this idea that might be a bit easier for some ppl.
Use the thumbs & index fingers of both hands to form a suitable window (round-ish or diamond-shape is fine, but not really important). Hold this window at arm's length and, with both eyes open, look at some distant object that fills up a fair portion of your window.
It is best and easier to use a small window rather than a larger window to do this test.
Check this website out for how to create the window with your hands.