How to tell Max CP?
You could do this with a protractor. Originally I thought you'd need to do an upgrade and measure the difference, but I was making it more complicated than it needs to be.
Thankfully, the bar is exactly half a circle, this makes things a bit easier with a standard protractor. I have assumed that the bar is linear, meaning that if the bar is half full, then the Pokémon is at half of it's max CP.
The bar on this example is 110 / 180 or about 60% full.
You can then work out your estimated max level with this formula:
max = CurrentCP / (angle / 180)
Example:
max = 72 / (110 / 180)
max = 72 / 0.61
max = 118
Based on the other answer, it seems that the max CP might change as you level up, so you might need to remeasure.
But again, a physical one would have to have the same radius as the image/app on your screen.
Nope, with a ruler (or any straight edge for that matter), you can use a smaller protractor. Just put the protractor so that it's middle is in the middle of the bar and place your straight edge as indicated by the white line.
Your Max CP of a Pokemon is also based on your level, it will increase if your level goes up. And the same Pokemon could have a different CP max. Some have more potential for growth than others.
So there isn't really a way to accurately determine what the max CP of a pokemon is.
You can calculate the exact maximum CP for each Pokémon, as long as you have all the correct data for that specific Pokémon.
Individual Values
The first thing you're going to have to figure out, are what the individual values (IV) are for each Pokémon you want to compare. Every Pokémon has 3 hidden IVs, one each for Attack, Defense, and Stamina. These values range from 0 to 15, and are used to calculate the Pokémon's CP.
You can use online tools such as the IV Calculator from Poké Assistant to find these values.
Base Stats
The next thing you'll have to look up, are the base stats for whichever species of Pokémon you're comparing. Each species has a set of base stats, which are also used to calculate CP.
You can find base stats all over the web, The Silph Road has a whole list of Species Stats
Level
To calculate the maximum possible CP, you'll simply use level 40 in your calculations. If you want to know what the maximum CP is for your current trainer level, you'll use your trainer level plus 1.5. For example, if you're level 20, you can only power a Pokémon up to level 21.5.
CP Multiplier
To calculate CP, you don't use the level directly, instead you have to look up the CP Multiplier for that level. Luckily, the CP Multiplier can also be found on the web, at sites like GamePress.
The Formula
The formula looks like this...
Max CP = Floor(((BaseAttack + IndividualAttack) * (BaseDefense + IndividualDefense)^0.5 * (BaseStamina + IndividualStamina)^0.5 * CPMultiplier^2) / 10)
As an example, I'll calculate the maximum CP of a 12/14/15 Pidgey if your trainer level was 20.
Max CP = Floor(((85 + 12) * (76 + 14)^0.5 * (80 + 15)^0.5 * (0.619399365)^2) / 10)
Max CP = Floor((97 * 9.4868329805051379959966806332982 * 9.7467943448089639068384131998996 * 0.383655573362403225) / 10)
Max CP = Floor((3441.0921526345817698060490793408) / 10)
Max CP = Floor(344.10921526345817698060490793408)
Max CP = 344
You can also easily calulate the absolute maximum CP, by simply using the CP Multiplier for level 40.
Max CP = Floor(((85 + 12) * (76 + 14)^0.5 * (80 + 15)^0.5 * (0.79030001)^2) / 10)
Max CP = Floor((97 * 9.4868329805051379959966806332982 * 9.7467943448089639068384131998996 * 0.6245741058060001) / 10)
Max CP = Floor((5601.9440442159961329825324916023) / 10)
Max CP = Floor(560.19440442159961329825324916023)
Max CP = 560