The only sure way to sex is to vent the fish. Depending on your tank size, you could add another Angel, but with caution. Angels are just as territorial as other cichlids, and yours will own the tank. So just putting another Angel in might cause a bit of a ruck. Try rearranging the decor, take your Angel out, put the new one in, and then reintroduce yours. But always have a plan B. Like arranging for the shop to take the new one back if things don't work out.