A good sponge if it's a Vanilla one, should be a light tan colour.(It depends on how you like your sponges too, but I usually prefer it a light tan color.) Chocolate sponge is a little harder to guess the colour.
But usually inserting a kebab skewer into the middle of the cake will let you know if it's done. If it leaves any cake on the skewer, it's not done. If it comes out clean, it's done.
Also touching the top of the sponge will determine if it's done too. If it bounces back it's done right. If it doesn't bounce back, it's not ready.

Hope this helps you out a little.