Heck of a situation here!!!
The first few things I would say about attempting to find out the pump's designed voltage would be:
1: Nameplate Amperes - make it pump water through the same size pipe which it's intended to use, then read the amperes with your clamp-on ammeter. Compare this to the rated FLA. In other words, push it near it's designed Horse Power.
This might also in a round-about way reveal the design voltage - such as if the design voltage was 230 VAC and it is connected to 115 VAC, loading it to the FLA will either stall the motor, or trip an overload on the winding.
Seems likely that if the motor is large [physically and HP wise], it would be on the higher voltage.
2: Setting of voltage selection jumper [if any].
Verifying the current setting on this jumper[s] position should answer the design voltage setting.
If two jumpers are used - connecting the windings in Parallel, then it is setup for lower voltage [115 VAC].
If one jumper is used - connecting the windings in Series, then it is setup for higher voltage [230 VAC].
This is only helpful if you can check, or test, the windings' terminations - typically where the motor circuit connections are made, and/or where the start switch can be accessed.
3: If a visual ID of the windings can be made, try to see what size the conductors on the run winding[s]. If smaller than #18, most likely designed for 230 VAC - all though this is kind of a long shot!!!
These items may help you [sure hope they will!!!], but they are not 100% accurate - except maybe the 1st one - whereas the motor would likely stall if trying to pump the rated amount of water, while supplying the motor with 1/2 the rated voltage. This test is something that you should not perform over more than 5 minutes, just incase a thermal overload occurs [windings overheat].
As you said, these typical use Squirrel Cage Induction motors can still run at 1/2 the voltage, under low and even medium load, and seem to be functioning correctly. It's just at the full load, or rated output HP, do things begin to show their ugly face.
** Now on the Q's about the starting Capacitor;
Some Split-Phase type motors of upto 1/3 HP - and possibly even upto 3/4 HP - can be started with just a "Resistance Split-Phase" type scheme. This uses the typical "Start" winding without a Series Capacitor. The Start winding has a larger Resistance to Reactance ratio with the main "Run" winding.
It is very similar in build to the Split-Phase Capacitor Start motor, where it has a "Start" winding placed 90 electrical degrees offset from the main "Run" winding, plus the "Start" winding is controlled via a centrufigal switch.
If the motor does use Capacitors [either for Starting, or for Starting, then Running], they might be located on/in the motor frame or enclosure. The typical Start Capacitor would be rated at 130 VAC max., even if used on a 230 VAC circuit - due to the fact that the Start winding will drop 1/2 the voltage - so the highest voltage possible across the Capacitor would be 115 VAC [when the motor is connected to 230 VAC].
Thing is that the physical size of a Start Capacitor would make it's enclosure very noticable.
One other type of Squirrel Cage Induction motor that might be used, and has no Capacitor [or even a switchable Start winding] would be the Shaded Pole motor.
These motors usualy max out around 1/6 HP, but maybe a few 1/3 - 1/2 HP models are floating around.
They have a typical loaded speed rating of 1450 RPMs, or 3000 RPMs.
Although they could be used for fluid pumps, they are commonly used for Air moving applications [e.g. Fans].
So, to sum it up - Maybe it's designed for 230 VAC, due to the color code [???], but that's not a great way of determining voltage since it's likely a multi-platform cord [can I use that term on power electrical stuff??
Guess that if no nameplate ID is available, the next step would be to pin point it through a series of tests. If you do go this route, contact me for the procedures and numbers.
Maybe [hopefully] another member can offer better suggestions than mine, which will be more accurate [and simpler] than my "Close to Smoke Test" procedures!!!
Sorry I can't give any better, or precise examples
P.S. Let us know the outcome of this one! It would be a great topic in a "How-To" area
[hint-hint to Bill