If a vst synth is recognized as stereo by Sonar can I always assume it is stereo.. for example if use 4front bass it appears as stereo.

Normally a Bass guitar is recorded in mono but I 'think' the guitars in kontakt and Play (Ministry of Sound) are stereo. (I'm away on holidays atm and researching a lot).

I'm probably getting confused. But was just wondering if some of these stereo plugins just send mono source down as stereo.

Yes, some do merely send the same signal to both channels. I know of no easy shortcut for determining if a synth's output is truly stereo or not, other than looking at the waveform or using a stereo width meter.
 
Your best bet for sampled bass is to take one side out into a mono track. Then you needn't care. The only time that doesn't work is if the synth/sampler is faking stereo using filtering or chorus, in which case one side alone might sound thin. But 99 times out of 100 you can successfully use one side out of any synth or sampler.

Thanks for that Bit,   I guess its a try and see if there is any difference.
 
 

bozdigitallabs has a free Panipulator that might help you with checking those things.

A lot of synths (other than sample playback ones) create a mono signal in the oscillators.  If there are no effects like ping-Pong delay or reverb, you'll get identical signals output in both channels.  It's usually the effects that would introduce channel differences (and of course the synth's pan setting).  If you have a sound that gets boring with just one channel is used, but you need a mono track, you can always slap Channel Tools in the signal path and sum the L and R to be identical.
 
With samples, it will depend of if they are mono or stereo to begin with, but the remedy would be the same.

Just out of interest .. when I was culling my vsts I noticed that there were three flavours of NI's Absynth.. Absynth, Absynth Stereo and Absynth Surround.

So is tge first instance of Absynth mono? Some guys on Kvr didn't even realize there was a difference..

Someone wrote this:

Absynth will open the synth with surround outputs (5.1 IIRC).
Absynth Stereo will open the synth with stereo outputs.
Absynth FX is for running external audio through Absynth's FX engine in stereo.
Absynth FX Surround is for running external audio through Absynth's FX engine

And this: The non-stereo version is not mono, but provides 6 outputs for surround sound.