How to determine amp rating ?

[Tat2]

OK; I've been asked a question by a friend of mine that I don't know the answer to, so I'm coming to you guys, who know more about these things than I do, with the hope that someone here can answer these questions.
Here goes...
My buddy is looking for a brushless motor for his TRX Rustler, and he's apparently been looking at some motors that don't list an amp rating (continuous or peak) in the motor specs. What he/we want to know is if you know some of the motor's basic physical specs (length and diameter of the can, Kv rating, # of poles, armature size, motor wire gauge) is there a way to determine, or a formula to calculate the amp rating of the motor? Sorry if that's the wrong question to ask, or if it's worded poorly, but I wasn't given much to go on either. While I know that an EagleTree or similar system would be the sure fire way to determine the answer to these questions, what I/we want to know is if you can determine a motor's amp rating, before actually buying the motor, just based on it's physical specs. Or is that impossible to do with such little info to go on? How does an OEM determine the amp rating range of a given motor of theirs? Is there a formula they use, or we can use, to calculate these things?

Here are the questions that were asked of me, in their original form:
"I have a few brushless systems; vxl, tekin and castle, but for the purposes of our discussion here I'll use the VXL as an example.

The vxl3s esc is rated for 200A continuous and 320A peak and the motor is rated for 65A cont, and 100A peak.

My question is; Is there a way to tell how many amps a motor is rated for based on the Kv or turns, among other physical charteristics like armature size and # of poles, for example? I see a lot of motors on eBay, and many don't list the amperage specs.

So, without knowing what the OEM rates a given motor for in terms of it's amp draw, is there a way to tell, or calculate that rating range? ...Burst or Peak?

Also if two motors were both 3500Kv, but one was rated for 80A continuous and the other was rated at 70A continuous, would that just mean that the 70 is just a more efficient motor and would deliver the same performance of the 80?"

Don't shoot me, I'm just the messenger. Any help, insight or answers that you guys can provide would be greatly appreciated. Thx!