OK, so is there a way to tell when the brakes are engaging vs. just using the power regeneration wrapped around the front axles?
Not sure what you mean by "the power regeneration wrapped around the front axles," but to expand on what canehdian said, think of it this way.
When the foot is on the accelerator pedal to accelerate or to maintain speed, electric current flows through the coils of the Volt’s primary traction motor to turn the shaft, and the motor’s shaft turns the wheels via the drivetrain to move the car down the road.
When the foot is taken off the accelerator pedal, that drivetrain connection between the wheels and the motor allows the process to be reversed. The momentum of the car is now turning the wheels, and, via the drivetrain connection, this then can turn the motor’s shaft, reversing the process and transforming the motor into a generator. Using the car’s revolving wheels to apply torque to turn the generator shaft consumes the car’s momentum, reducing the speed of the car. Adjusting the electrical circuits modifies the rate of generation (e.g., D, L, paddle, or variable via the brake pedal), which increases or decreases the torque needed to turn the shaft, and thus how fast the car slows down. Braking regeneration is a convenient and effective alternative to friction braking.
Braking regen has the added benefit of generating electricity to recharge the battery, which recaptures some of the battery power used to accelerate the car (from a standing stop, or from one speed to a higher speed).
Battery power used to maintain speed is irrecoverable (i.e., the amount of regen you obtain by taking the foot off the accelerator pedal does not depend on the length of time you have been driving at that speed).
I’m not sure if both motors are used as regen generators when the Volt is in two-motor or power split configuration, or if the regen operation is limited to the primary traction motor only (yes, regen happens in Extended Range Mode, too). As Sean Nelson points out, increasing the number of regen generators doesn’t increase the amount of regen captured. It just increases the number of generators used to recapture some of the power used to accelerate the car.
Maximum fuel efficiency is achieved when the use of fuel to accelerate the car is minimized. Note that using L or the paddle instead of D to get closer to the restriction point before slowing down (e.g., a traffic light that may or may not turn green by the time you get there or a stop sign where you must stop) may create regen at a higher rate for a short period of time, but it may also be preceded by a longer period of maintaining speed before you reach the slow down point (i.e., irrecoverable power consumption may offset any higher rate of regen gain).
The Volt’s rear wheels have no connection to any motor/generator, and no braking regeneration is created there.
Downhill regen is similar to level-ground regen, but gravitational energy, rather than battery energy used to accelerate the car, is providing the car’s momentum. The downhill momentum uses the wheels to turn the generator shaft, can continue until the car reaches the bottom of the descent, and the process provides a braking effect on the car.
Gravitational forces complicate regen analysis. Much uphill driving is done while maintaining speed (irrecoverable battery power use for the horizontal driving portion of the power consumption), and much of any power used to accelerate uphill will also be lost because gravitational pull replaces some or all of the braking regen whenever the car slows or stops. This makes it more difficult to determine the portion of the battery power consumed for the vertical portion of the bottom to the top of the hill driving that may subsequently be recaptured by downhill regen. The analysis is further complicated by whether or not the car can continue "coasting" as the terrain levels off (i.e., no stop sign of traffic light at the bottom) until power is again needed to maintain speed, and whether or not the terrain is rolling hills, whereby the bottom of one descent is closely followed by another ascent.