The RAF and the Army do not wear white gloves as part of their uniform, so they will salute with their palm facing forward.
If you are not sure who is your superior officer based on your rank, you can use this chart here to determine who you should be saluting in the RAF and the Army. In the RAF, a Corporal will always salute an Air Marshal, for example, and in the Army a Private will always salute a Major.
In the Navy, officers can salute with their left hand if their right hand is engaged or occupied.
If you are not sure who is your superior officer based on your rank, you can use this chart here to determine who you should be saluting in the Navy. For example, in the Navy, a Petty Officer will always salute a Lieutenant.