Cremello is chestnut plus two copies of the cremello gene. Perlino is bay or black plus two copies of the cremello gene. You can't always tell the difference by looking. White is a whole other ballgame.
I have always understood it that a cremello looks like a VERY pale palomino and a perlino has the same body color but with reddish points (mane, tail, ears, legs) As for white, I believe the only way to have a white horse is a maximum white paint. All other "white" horses are generally faded grays. I agree, the only way to tell for certain is to have them tested.
Genetically they are both double dilutes. So a 'chestnut' with 1 dilute would be a palomino. A double dilute would be cremello. A 'bay' with 1 dilute would be a buckskin. A double dilute would be a perlino. Does that make sense? By "white" are you talking about a grey? Grey's are an entirely different genetic color. They are born dark and gradually fade out as they get older until they are white.
Depending on the individual horse, they can be very difficult to distinguish. I do think that cremellos tend to be more of a yellowy color rather then the perlinos which sometimes are more of a reddish or brownish tint. A lot of times they just look like stained dirty grey horses to me. :)