CrossRoads:
File:Upload Using Programmer, loads the sketch using the AVR programmer connected to the ICSP pins.Most bootloaders will make the L LED flash - maybe continuously with three flashes every few seconds, or maybe just a couple flashes after a reset.
I haven't powered up a freshly bootloaded Mega in a while, I don't know what the current bootloader does.
I believe the bootloader LED behavior is to continuously blink three flash bursts every so many seconds IF there is no sketch in flash memory to jump to, if there is it jumps to the sketch after just one three second burst. Keep in mind that different bootloaders for different boards over the years can have slightly different LED patterns.
Lefty