parted or fdisk might be usable here...
For example:
```
# parted -s /dev/sda print >/dev/null 2>&1
# echo $?
0
# parted -s /dev/sda1 print >/dev/null 2>&1
# echo $?
1
```

If you wanted to be pure python, you could read in the first 512-bytes of the device and seek in to the proper offset to see if there's a partition table...
http://en.wikipedia.org/wiki/Master_boot_record