I would think a combination of visual examination and voltage readings would do the trick.
If there's no neutral conductor on the service bonded to ground, it must be delta.
If one conductor is grounded and there are only two other service conductors, it will be a corner-grounded delta, and you'll get the same voltage reading between the two hot legs and from each hot leg to ground.
If there are three conductors plus a grounded neutral, it could be wye or 4-wire delta.
Wye will show the same voltage from each phase to ground; 4-wire delta will show the characteristic "high leg" voltage on one phase.
