follow these rules

This procedure leads to the following general result:

*1*If the degree of the denominator exceeds that of the numerator, y = 0 is a horizontal asymptote;

* 2 *If the leading coefficient of the numerator is a and the leading coefficient of the denominator is b and their degrees are equal then is a horizontal asymptote;

* 3 *If the degree of the numerator exceeds that of the denominator, there is no horizontal asymptote.

http://www.math.csusb.edu/math110/src/rationals/hasym.html