Hooks way is the easiest way, You can also jack up 1 front wheel and with engine shut off(real important for safety) and the tractor in gear turn the jacked up wheel by hand, If it turns by hand the 4 wheel is disengaged , If you can not turn it the 4 wheel is engaged, Other ways are if you have an area of soft sand you can tell, In the soft sand in 2 wheed drive the rear wheels may slip and when the 4wd is engaged the front wheels will pull.