Assuming this is a Zavoli pulsed injection system...
There are a few basic checks you can do;
Pull the vacuum pipe off the reducer while it's running on LPG, no gas should come out of this port on the reducer.
Use a bit of pipe in ear as a stethoscope to listen to each injector. They should all sound similar, any that are much louder than others or making a fairly pronounced metallic noise are probably shot, although they could be shot even if quiet! Being a Zavoli system it will either have Zavoli's own injectors (either individual injectors or mounted on a rail with individual units easy to identify) or Matrix injectors (more of a square block, individual workings not easy to identify except for seeing gas outlets). Matrix are by far the better type in terms of both longevity and performance, they may further be identified by the single big square electrical connector block,
If you have a suitable scanner, check fuel trims are good and similar on either fuel. And/or check lambda readings under various engine operation conditions.
At 11 years old and many miles might expect even Matrix injectors to be a bit worn.
Even if you had the interface for the LPG system, diagnostics capabilities via the old Zavoli ECU will be limited, it is unlikely to have capabilities of more recent LPG ECU's such as ability to switch individual cylinders between LPG and petrol. Though a cable would allow you to adjust calibration of the system of course. For more advanced checks on parts such as injectors a more recent ECU could be temporarily swapped in.
Hope that helps.
Simon