know what (you) are doing

to have the necessary knowledge or experience to do something.

Mountain climbing is a dangerous sport, so I hope he knows what he's doing.

• a rainy day fund
• a rake-off
• a rap across the knuckles
• a rap on the knuckles
• a rap over the knuckles
• a rap sheet
• a rat fink
• a rat race
• a raw deal
• a ray of sunshine
• are all over
• a red cent
• a red eye
• a red herring
• a red-letter day
• a reign of terror
• a Renaissance man
• a rent boy
• are someone all over
• are something all over
• a revolving door
• Are you a man or a mouse?
• argue the toss
• argy-bargy
• a rich seam