There really isn't an exact way to tell unfortunately.
The closest you can get is to open the default signature template.
It has a field for selecting which version of signatures you want to look at.
Look at the last IDS sensor appliance version listed.
The S level is typically the signature update level installed.
Example:
If it lists 3.0(4)S16 as the largest IDS sensor version then S16 is installed.
Occasionally, however, this is not accurate.
For example if the latest IDS sensor version is 3.0(4)S17 then the S17 is installed, but if the latest is 3.0(5)S17 then S17a is installed.
3.0(4)S17 -> S17
3.0(5)S17 -> S17a
