Hi again.
The clearly smaller size compared with Common and Pallid Swifts, voice, and throat not so whitish as in those species are the best features to identify the Plain Swift (apart from much darker coloration in comparison with Pallid). The best occasions to see well such differences are, naturally, when you have at the same time Plain, Common, and Pallid Swifts, e.g. during the spring or early summer. Common Swift breeds in some islands of the Canaries, but its distribution is not well known, while colonies of Pallid Swift are mostly located in coastal places and the species is normally scarce and local.
Best regards.