Listen for USB "TX signal received" response on RX Is there a usb signal I can send on TX that will generate a response from the computer on the RX only if the computer is active/not a wall usb power supply? I'm trying to use the Remote library when a button is pressed and sometimes it won't work because the computer is off/asleep/etc and in those instances I would like the arduino to behave differently but because I have no way of knowing if the USB power is coming from the wall or passively from a computer that's off or actively from a computer that's on, I can't know what behavior to take. 

To be clear about the application in mind: Right now I have a device that sends keyboard signals to my computer using a modified version of Stefan Jones' modified USBAPI.h and HID.cpp files. It works fine but if you restart the computer you have to manually unplug and replug in the arduino to get it to work again. This is necessary every restart, is rather annoying, and adds unnecessary wear to the connector. This is also necessary if the arduino is connected before the OS has loaded. Additionally there are some LEDs on the device (not talking about the diagnostic lights on the arduino itself) that stay lit up at night even when the computer is off. If I could detect when the computer is off/unconnected it would be trivial to fix this.
If an MCU which has a built in USB interface (such as the ATmega32u4 on a Leondardo) is properly configured and connected to an operating USB host, it will receive a "Start of Frame" packet every millisecond.  I'm not very familiar with the details of the USB engine on that particular chip, but generally you can detect this as an interrupt, or as a change in a peripheral status register bit.

Going a step further, in the Arduino API an if (Serial) test will determine if the board is connected to a host which is in an active CDC-ACM "serial" session with it, ie, one that has a serial terminal or other program talking to it.  Again, this only applies to those that implement USB within the chip on which the sketch is running.

Chances are your problems can be fixed by enabling the AVR watchdog timer at one of its longer limits and resetting it in the USB ISR found in USBCore.cpp Then if the the USB goes down (ie, the usual every millisecond SoF interrupt fails to happen for several seconds) your Arduino will reset.  In the simple case, it will keep resetting as long as it has power and the USB is down, but you can put some startup logic to wait for the USB while resetting the watchdog if that bothers you.  See for comparative example the implementation details behind the if (serial) and create something similar for your chosen USB mode.