To easily identify real numbers in domain and range;
if you have a given function, then make a table for x and y, then plug-in in your equation to find out x value as well as y value..
all numbers that can be found in the table are now your domain (x) and range for (y) either it could be infinity or all real numbers, if you wish to graph the points in  a plane, you will see those real numbers are in your sketch, those who are no belong in the sketch are not part of the domain and range...