A white gold engagement ring will be longer lasting that a normal gold engagement ring. This is simply because white gold is more durable and harder than normal gold. 24 carat pure gold is soft an malleable and would wear away within a few short years. By adding silver, zinc, copper and sometimes nickel to the gold one not only creates a longer lasting ring but also changes the color depending on the actual mix.
A white gold ring would actually look off white and possibly even gray. Many white gold engagement rings therefore are coated in either platinum or rhodium which is silvery in appearance and also very hard wearing.
So if you get one of these rings how do you know it is white gold? You can have a jewellery tech this for you to ensure it is the genuine article. When you get the ring it should also come with a certificate of authenticity and warranty that the ring is exactly what the jeweller says it is and with the option of return if found to be something other than what was described.
As the ring would normally come with a diamond this should not be too difficult as the diamond should also come with a certificate of authenticity and have been appraised.
A white gold engagement ring does have a unique beauty and, with the right diamond, will enhance the hand is has been placed on.