A good way to tell is the presence or lack thereof of primitive markings. A dun will show some or all of the primitive markings - Webbing, leg barring, wither barring, a dorsal stripe, a wither/shoulder stripe. A buckskin does NOT have primitive markings. they may have countershading as demonstrated above but if a true dorsal stripe or leg barring (The two most common primitive markings) are present, then you can be sure that the dun gene is present. The colour produced by dun and buckskin also differ a little - A trained eye can usually tell.
It is harder to tell if the dun gene AND the creme gene are present together. Testing is the only way you can know for sure.
As Haviris mentioned, the cream gene and the dun gene act differently when there are two copies present, known as being homozygous. With cream, it produces what is called a double dilute - The dilution is maginified, creating Cremellos (The most widely known double dilute - Pink skin, blue eyes, creamy hair - Two copies of cream on a red base coat) Perlinos (two copies of cream on a black base coat + agouti [Also known as bay] who have pink skin and amber coloured eyes) and smoky creams (two copies of cream on a black base coat, no agouti, who look similar to Perlinos).
On the other hand, a horse who is homozygous for dun cannot be identified just by looking - Testing is the only way to know.
Testing is quite important to a lot of breeders. A horse who is homozygous for a certain colour has a 100% chance of passing that colour on to offspring - So the owner of a homozygous dun stallion can give mare owners a 100% colour guarantee. However the same stallion with only one copy of dun has only a 50% chance of passing the colour on to any offspring, which aren't as good odds. Testing can also reveal colours that 'hide' under other colours or are only activated when another colour is present, such as pearl - Unactivated unless the horse also possesses the cream gene, yet can still be passed on unknowingly to progeny. Hence, testing is a great tool for breeders when promoting their stallions or breeding their mares.