The QRS axis is the mean direction of electrical current propagation through the myocardium.
the QRS axis

There are several methods to determine this direction.

The isoelectric lead method

The AXIS is the chief direction of QRS propagation in the vaguely coronal plane.

To find the Axis in two steps, one must focus on the left side of the ECG.

RAD

1) Select the lead with the most isoelectric trace; i.e. the lead in which Q and R waves are of the same height. The axis will be perpendicular to that lead orientation. You need another lead to determine which direction the QRS is pointing in.

2) Select the lead which is perpendicular to the isoelectric lead This is the lead which will give you the direction. In the ECG presented here, the isoelectric lead is aVF; its perpendicular lead (lead I) is negative, suggesting the impulse is travelling away from Lead I – which makes this a right axis deviation.

Which leads are perpendicular, you ask?

the perpendicular leads in the ECG

Lead I is perpendicular to aVF

Lead II is perpendicular to aVL

Lead II is perpendicular to aVR

The quick look method

Looking at aVF and Lead I alone will usually give you the axis.

Normal Axis

-  Lead I is POSITIVE
-  aVF is POSITIVE

Left Axis

-  Lead I is POSITIVE
-  aVF is NEGATIVE
-  Lead II is NEGATIVE

Right Axis

-  Lead I is NEGATIVE
-  aVF is POSITIVE
-  Lead III is POSITIVE

No Mans Land

-  Lead I is NEGATIVE
-  aVF is NEGATIVE

The Quickest Look Method

Just cheat, all's fair.

The ECG machine will frequently print out the axis.

- Normal axis is -30 to 90.

- Left axis is anything more negative than -30

- Right axis is anything more positive than 90.

References

From "the ECG made easy", by Hampton (2003), and ECGs shamelessly stolen from Life in The Fastlane without any sort of permission, but in the non-commercial spirit of free education