If it's logical or single or double,
if theImage(y, x) == 0
Or, it if's uint8 instead of logical or double or single, if it's 0 it's black, if it's 255 it's white, and if it's in between it's a shade of gray.
Be careful of the common mistake Nitin made where he said I(x, y). The row (y) is the first index and the column (x) is the second coordinate, so if you want the value at some x,y coordinate, it would be I(y, x).