The best altitude alert is good CRM, and good airmanship which is enhanced and informed by a good, thorough briefing by both pilots prior to departure or arrival, and then good scan, and good monitoring by the monitoring pilot.
The second system can be fitted in aeroplanes fitted with EGPWS that has a GPS feed, either from the aeroplane's fitted GPS or from a GPS card in the EGPWS. There is a database that has all the currently known (by the manufacturer) transition altitudes, and levels for all the currently known, and included in the database airports in the world. The accuracy of the information that your EGPWS gives you relies on your airline updating the database in the EGPWS. In the case of Honeywell systems the updates are available at no charge for your airline to download from Honeywell, and to update as they are released. The information in the data base is gathered by Honeywell from AIPS and so on, so relies heavily on the AIP being updated and amended correctly by the National Authority, so the first paragraph now applies again.
The altitude alert is only an aid to situational awareness, and will never beat good airmanship, and good flight deck practices.