Open a command prompt and execute:
wmctrl -pl. This will give you a list of windows and their process IDs. It will look like:
0x03800008 0 2138 [your_host_name] unity-panel
0x0380000b 0 2138 [your_host_name] unity-dash
0x0380000c 0 2138 [your_host_name] Hud
0x0280000a 0 2213 [your_host_name] Desktop
0x0380000f 0 2138 [your_host_name] unity-panel
0x02c00078 0 2230 [your_host_name] Inbox - Mozilla Thunderbird
The last column is the window title. Match that to the application you are interested in.
The 3rd column is the Process ID. To find out the application that started that particular process ID use
cat /proc/[the_proces_id]/cmdline. In the case of Thunderbird I used
cat /proc/2230/cmdline and it produced
Thunderbird is not a very good example because it uses a stub in /usr/bin which starts the real application located in /usr/lib/thunderbird/.
Just starting /usr/lib/thunderbird/thunderbird however may or may not work and I do not recommend it. Instead you need to find out which program in /usr/bin (or maybe /bin) started the program you are interested in. To do that pass the program name to the program which, i.e.
which thunderbird. It will give you the real program name, i.e. /usr/bin/thunderbird.