<Mousing over my minimap, there is a different named mob nearby 'Grizlak'.>
Every new questing point that you head to with enemies, scan over your mini map with your mouse, and check for any unique names. Rare mobs will be named differently to the majority of creatures around it. When you mouse over the unique name, head towards that direction and hunt down that rare mob!
<Misty doing a fine job distracting the mean rat!>
Along with the above tips for hunting rares, remember to keep your eyes open, and if there is a mob that stands out from the rest (larger in size, slightly different appearance) this may also be worth selecting to see if the silver portrait comes up around it's nameplate.
Rares will also award you with more XP than standard mobs, and usually a better than average piece of loot.