On a dry and fine evening just after sunset, you have gone for buying a new shirt from a show room alongwith your friend down a lane. Lights are on in the streets and in the shops as well.
Due to some reason, your friend took few quick strides forward and he was ahead of you by 10 - 15 feet at which both of you happened to be infront of a shop/show room which has shirts displayed (needed by you) and also one of it, which is adjacent to the glass sheet and at around 6 - 7 feet away from you stole your heart and you decided to buy it.
At this moment, being sharp-eyed, you noticed a stain (or spot of dark color on a light back-ground) of considerable size and you are not sure where it is - on the shirt that was being displayed OR on the 1/2" thick crystal clear glass.
Here comes the actual question - without moving a single inch from your place, how can you decide if the stain/spot is on the shirt / on the glass sheet ?