Hello Joe and thank you for the post.
We have number of options for evaluating injectors and you may have read a couple of posts on here that talk about AC coupling of the fuel rail pressure sensor and injector "leak off" evaluation using the WPS500 pressure transducer. (Ideal for Toyota etc. where like you mention, injectors are under the cam cover)https://www.picoauto.com/support/post31629.html?hilit=spill#p31629https://www.picoauto.com/support/topic9829.html
I believe both these threads are of real value with great contributions from all.
We therefore have a number of options at our disposal when evaluating injectors in a reasonably non-intrusive fashion utilising.......
The current clamp and test leads to evaluate the circuit and the "load" applied by the injector
Fuel rail pressure AC and DC coupled utilising the scaling and filtering features of the scope to evaluate the effect of each injection event on fuel rail pressure (Pulsations and induced voltages are present in this signal)
The pressure transducer for leak off tests to evaluate each injectors contribution to back leakage pressure pulsations
I hope this helps and any other techniques/tips would be most valued here given the amount of injectors we see and the costs involved in their replacement.