Can an evil character cover there alignment?

Is there anyway for an evil NPC to cover up there alignment if a player attempts detect evil?

I am working on a Silver Dragon antagonist. This NPC will appear to the players as a number of human NPC’s setting tasks, missions, appearing as a friend. The dragons goal is to attempt to prevent an ancient doomsday prophecy coming to fruition. Each spell it learns will become branded to its scales in a series of runes which in human form will take the form of tattoos so over time the players may be able to work out these different humans are connected in some way.

The Dragon will determine the only way to stop this prophecy is to enact an ancient ritual involving it sacrificing itself to destroy all intelligent creatures with evil or chaotic alignment in all the planes of my world therefore becoming the very prophecy it was seeking to prevent. At first I thought the dragon would remain good as it believes its aims are good, but, I now see it will have to have its alignment shift as the campaign progresses and it becomes more convinced that mass genocide is the only way to save the world from lawful good at the start, to chaotic good and then chaotic evil.

Is there a current way for this dragon to hide its true alignment from any magical check or am I going to have to create a way for it to do this? Possibly one of the spells the party hunt out for it.