every pet has the potential to be a rare, iv caught over 50, it just takes patients.
fastest way is to engage in battle, an if it isnt rare, forfit an find the next one. when you forfit the bad quality pet despawns, so hopefuly when it response, it will be rare.
One awesome way to determine a rare, is an addon i found called Pet Battle Quality Glow addon which can be found on wowinterface.com
the addon changes the color of the name text for all pets (yours, wild pets, an even pvp pets) when encountered. Showing you wish ones are poor, common, uncommon an rare!