[RPG] What are some resources or tips on dragons as PCs

Challenge rating

...shows the average level of a party of adventurers for which one creature would make an encounter of moderate difficulty.

This makes challenge rating a better instrument to measure how monsters stack up against a party of adventurers than how monsters stack up against each other.

A more precise instrument to measure how a monster-on-monster battle may run is effective character level (ECL). The Draconomicon lists the lowest-Hit-Dice young red dragon as ECL 19 and the lowest-Hit-Dice young copper dragon as ECL 15. Using these figures (instead of the creatures' challenge ratings or Hit Dice or whatever) makes the aforementioned young copper dragon's encounter with the young red dragon very difficult (DMG 49), which, as you've noted, it is.¹

But even this slightly more precise instrument is still a club not a scalpel. Determining which monster wins in an unseen monster-on-monster fight should be the DM's call and used to further the plot, making excuses for the lower-powered monsters when necessary to enhance verisimilitude (e.g. "Yeah, the copper rolled nothing but critical hits and the red failed every saving throw--it was amazing; it's too bad you missed it").

"But what are these creatures' actual CRs?"

Dragons' CRs are far too low if all their strengths are played to--each is, at least, a sorcerer engine in a dragon chassis, after all, and sorcerers are already among the game's most powerful classes--, but dragons are much closer to their printed CR if played like big, meaty melee monsters with the default feats from the Monster Manual (e.g. no Rapidstrike et al., no Shock Trooper, no Travel Devotion). It's the DM's task to make sure that the dragon and the environment in which the dragon's confronted are appropriate to his PCs' abilities instead of either slaughtering the party or allowing a legendary beast be unceremoniously assassinated (unless that's the goal).

A customized-by-the-DM wily young copper dragon that efficiently uses all of its resources can, certainly, defeat a straight-from-the-Monster-Manual young red dragon that's down on its luck, hungry, caught unawares, and used to biting everything to death... but either of those could be an EL 7 encounter. It's part of the DM's job to evaluate each monster to determine its suitability. Encounter design, unfortunately or not, is more art than science.

1. That these ECLs are likely excessive is another issue entirely.

By default, no

That is; Dragons (or any other creature types that I know of) have no default abilities that allow them to see the creature type of any other creature type, nor can they sense transmutations or illusions, they don't have Truesight (unless it says so in their entry, but I don't think any Dragons have it by default), so they don't have any special information.

But Dragons are old, powerful and usually paranoid

So any specific dragon might very well make sure that he can spot other creatures being disguised or trying to fool him. Good odds that a Great Wyrm will have an item that gives him True Seeing permanently to spot other trying to trick him. Although this still does not let him see a creature's Type, it will show him other dragons in hiding.