AFAIK there is no way to "find" other running installers. At least not on every system and every installer. The only good way is to use an installer which supports that like windows installer (but also that only checks for other running windows installers).
A probably working way would be to use a list of process names of installers and check if one of them is running.
Another way found as answers this question is to check a registry key (1) or use the windows api (2). But this two ways would only work on windows.
For ways to check if a process is running you could use the python module psutil. An example which prints all process names would be:
import psutil
for p in psutil.process_iter():
print p.name