Left axis deviation occurs when the QRS axis falls between -30 and -90. There are a variety of causes, including left anterior fascicular block and left ventricular hypertrophy. See the ECG basics section on determining axis for details.
If the QRS is upright in lead I and downward (negative) in lead aVF, then the axis is between 0 and -90 degrees, likely left axis deviation. However, in this scenario the axis could fall between 0 and -30, which is technically a normal QRS axis. To further distinguish normal from left axis deviation in this setting, look at lead II.
If lead II is downward (negative), then the axis is more towards -90 and left axis deviation is present. If the QRS complex in lead II is upright (positive), then the axis is more towards 0 degrees and the QRS axis is normal.