[Chronon]

1) You can see a partner's resupply depots if you go into the "Empire Status" window - the button at the top with the crown - and click on the Empire Options button. From there you will get a list of things to toggle on and off, and you'll want to toggle on the option that says "see resupply depots..." Once you've done that and exited back to the systems window, you will see little letters just below and to the right of your planets and your partners' planets. "R" stands for resupply depot, "Y" for space yard, and "S" for spaceport.

2) Not that I know of, but there may be an intelligence project that will do this for a targeted planet.

3) Planetary capture can be tricky in strategic combat. I have had some success with fleets set to "capture planet" as their strategy. But it's important not to have overwhelming firepower, or you will glass the planet with your first shot. Theoretically, once all the weapons platforms are destroyed, the fleet will stop firing and the transport will drop troops. But if all the population is killed in the first volley, there is nothing left to capture. One other thing is to avoid missile ships in the planetary invasion fleet. What might happen is they fire and move away, meanwhile your direct fire ships destroy the weapons platforms, and a turn later the missiles come in and wipe out the planet.