Hello , how can somebody find if a nano has old (atmegaboot) or new (optiboot) bootloader?
try to upload
install the boot loader yourself then you know what's there and stop wondering about it 
Do this:
- Power the Nano board (e.g., connect it to your computer with a USB cable).
- Press and release the button on the Nano board marked "RESET".
- Watch the behavior of the LED marked "L" on the Nano board immediately after you release the reset button.
| # blinks | bootloader |
|---|---|
| 0 | none |
| 1 | ATmegaBOOT (AKA "Old bootloader") |
| 3 | Optiboot |
Note that the LED might also exhibit some other blinking behavior depending on which application program is running on the board. For example, a new board will usually have a repeated blink program running.
So (?). if select "wrong" bootloader in tools, the sketch will NOT be uploaded?
I update all nanos with the Optoboot bootloader as soon as I get them, even the clones get updated, this way I can just grab one and don't have to worry about what version I am using.
ok, agree 100%. What happens if the nano is intalled 5 years ago?
it is not. you get a common upload error. "out of sync" I think
thank you