I think it depends on what the plugin reports. Most have "SC" added to the name or something.

I had a quick look and it looks like SideChain (for Cakewalk Compressors/Gates) is present on:
 
Direct X:
Sonitus:fx Compressor
Sonitus:fx Gate

ProChannel:
PC4K S-Type Bus Compressor * (default on Bus)
PC4K S-Type Channel Compressor
PC4K Expander / Gate

ProChannel and VST3:
CA2A T-Type Levelling Amplifier
 
* Note: the PC4K S-Type Bus Compressor appears to be one OR the other with PC76 U-Type Compressor (No Sidechain - default on Track)). You must remove or replace for the other to be 'choosable'.
Perhaps someone can confirm/explain? (I'm assuming due to no 'need' to compress multiple times (?)).
(I'm still using X3, own Platinum yet to install) 

ShellstaX
 
* Note: the PC4K S-Type Bus Compressor appears to be one OR the other with PC76 U-Type Compressor (No Sidechain - default on Track)). You must remove or replace for the other to be 'choosable'.
Perhaps someone can confirm/explain? (I'm assuming due to no 'need' to compress multiple times (?)).
(I'm still using X3, own Platinum yet to install) 

These two compressors were part of the original ProChannel which only allowed one instance of each type of processor. This was changed with X1 Producer Expanded. Unfortunately, these plug-ins are still stuck with their old design.

There is a pretty good video on sidechaining in http://www.cakewalk.com/CakeTV/SONAR-University/Go-Deeper
The send names can be a little cryptic.
 

Thanks for the explanation scook - makes sense.
 
Something from another post of mine ...
 
This is the best SideChaining video I've seen.
 
It's aimed at Prochannel - but things should be transposable to Sonitus Compressor / Gate. (I've pumped the mix through Sonitus)
 
It shows 4 techniques:
0:04 - Rhythmic Gating
2:14 - Vocal Ducking
3:28 - Kick to Bass
4:31 - Pumping the Mix