How, in a plugin, can I check that a User's "Favourite Entries" field contains a specific Entry?
You can avoid looping by getting an array of the IDs of the favourite entries, then check to see if the specific Entry ID is in the array.

$favouriteEntryIds = $user-&gt;favouriteEntriesField-&gt;ids();
if( in_array($searchingForThisEntryId, $favouriteEntryIds) ) {
    ... the Entry has already been favourited ...
}