If this is an ending diagonal, we should see price re-test sub wave (3) high (usually higher than wave (3) but should not breach the origin of blue wave 1 or a) then a sharp decline fall below the end of sub wave (4).
Since the reversal after ending diagonal usually very quick and sharp, it could be difficult to enter the trade manually, we could probably place a stop sell order just 1 point below the end of wave (4) (only if we see the wave (5) rally first).