The order of dragon difficulty

Post main quest dragons still spawn at least for the bounty quests given by stewards. I came across a couple out in the wild but in previously undiscovered locations so no guarantees they will still random spawn.

Edit: Scratch that, just got an unpleasant surprise in riften they still spawn everywhere.

According to the wiki yes, there is a way to tell by their names:

• Red dragons: These dragons have appeared in Skyrim after the emergence of Alduin. Their bodies have a greyish tone with a light grey underbelly. Yol (fire) is their breath attack.

• Green dragons: These Dragons have appeared in Skyrim after the emergence of Alduin. They have black skin with a distinct green glow. Commonly found in forested areas, they can use the Raan Shout (animal alligiance) to call nearby animals to their aid.

• Frost dragons: Frost dragons appear in Skyrim after the emergence of Alduin. Frost dragons are white with massive protruding black spikes coming out of their back. They predominatly use Fo (frost) breath that deals damage, slows the player down, and reduces stamina.

• Blood dragons: Blood dragons appear in Skyrim after the emergence of Alduin. They are recognized by their green-grey, crowned head, and large fan formation on the end of their tail. They become common at level 15-16, and use Yol (fire) and Fo (frost) breath attacks.

• Elder dragons: Elder dragons appear in Skyrim after the emergence of Alduin. Orangeish in color can use either Yol (fire) or Fo (frost). These attacks are generally strong and debilitating. Their tails can rend a forcefull hit and have the spade shape at the tip.

• Ancient dragons: Ancient dragons appear in Skyrim after the emergence of Alduin. These dragons have a tremendous amount of health and can use either Yol (fire) or Fo (frost) as the Elder Dragons do. Just as Elder Dragons they have a spade shaped tail tip.

• Skeleton dragons: Skeleton dragons appear in Skyrim. They are usually in dungeons, caves, and any underground structure with draconic lore. Although flightless, they are still dangerous. One such dragon appears inside Labyrinthian. These dragons don't have dragon souls.

Dragons that use fire based attacks are 50% more resistant to fire based attacks although 25% weaker when faced with frost based attacks. Vice-versa for frost breathing dragons

Using those clues, you can try to discern what type they are before you are fully engaged but most dragons seem to be of either type, so there's no easy way to tell unless you spot them fighting something else.