Check out this solution.

FieldExists for ArcGIS 10 Python

Check out this function from by Bjorn Kuiper to test if a field exists :

def FieldExist(featureclass, fieldname):
    fieldList = arcpy.ListFields(featureclass, fieldname)

    fieldCount = len(fieldList)

    if (fieldCount == 1):
        return True
    else:
        return False

with the following example of use:

    if (not FieldExist(myFeatureClass, "myField")):
      arcpy.AddError("Field 'myField' does not exist in " + myFeatureClass)
      sys.exit()