In the original game, it was only Battleships which would attack your base, and the player would always get to see the battleship as it approached the base, so the player would know who was attacking them. However, in modded games, attacks can be instant, done with any ufo, and the player doesn't get to see this crucial info when making a decision of whether to try to shoot down the craft or just let them attack.
I suggest that, when the base defense screen is shown, the player should be shown the name of the enemy ufo and, if Hyperwave Decoder is present, the alien race as well.
The logic behind it is that, even if it's a surprise attack, if the player has enough time to choose between shooting or letting them enter, then they should also have enough time to get a glance at who's attacking the base before making that decision.