Usually the airlines themselves designate certain fields "captains only" in their operations manuals. Aspects like strong and gusty winds, steep approach paths or difficult terrain (mountains) could lead to such a classification. Prominent examples are Funchal, Gibraltar, Innsbruck and the old Hong Kong Kai Tak airport.
If an airline does use such a policy (and I quite understand why it would), how do the F/Os gain experience of operating into those challenging airports so that they can handle them when/if they become captains? Are they flown in the simulator? Are they covered on the command course?