Can you ask another golfer which club they used?

Advice:

• Asking for it from anyone but the caddie, or giving it to another player in the competition: the general penalty.
• Not advice: where things are on the course, the distance from one point to another, the direction of the wind and the Rules themselves.
• A player must not touch another player's clubs or bag to find out what club was used.
• Partners in foursomes and four-ball may advise each other.

What a caddie may do:

• Carry and handle the clubs, search for the ball, give advice before the stroke, smooth bunkers, repair damage on the green, attend the flagstick, remove loose impediments and movable obstructions and clean the ball.
• Mark and lift the ball on the putting green without being asked.
• Lift the ball elsewhere only when the player authorises it each time.

What a caddie may not do:

• Drop or place a ball in taking relief, or replace a ball the caddie did not lift.
• Decide to take relief. The caddie may suggest it, but the player decides.
• Concede a stroke, a hole or a match.
• Stand on the line of play behind the ball once the player starts to take a stance, to help with aiming.
• Set an object down to show the line.

Also:

• A player may have only one caddie at a time.
• A breach of the Rules by the caddie is a breach by the player.

The Rules describe deciding strategy and tactics as "a fundamental challenge for the player". The caddie is the one exception, and even the caddie has to step away before the swing.