Equipment does exist for external validation of stimulus timing and synchronicity through opto-detectors and microphone arrays - For example see here - https://link.springer.com/content/pdf/10.3758/BF03195575.pdf

What is the task though? For a lot of people, using psychtoolbox/eprime/psychopy is accurate enough in their production times.

You can measure when the screen is being drawn by using a photo diode. Our lab have a white square in the left most top corner of the screen which is flashed along with the stimulus. A photo diode is placed in front of it and the signal is sent into our data acquisition system. The timing of the stimulus onset is locked to the rising edge of the photo diode signal.

I came across this photodiode circuit (Fabricated in India). https://www.sunrom.com/p/light-sensor-module-photodiode

I have not tested this myself. Please check it out and see if it help. And do let me know if it works

Thank you Huw Swanborough. The experiment involve multisensory synchrony judgment task, stimuli(Audio-Visual) presented with variable SOAs ranges from 0 ms to around 300ms with 16ms(approx) steps and participants asked to judge whether two events are in synch or not. I use psychtoolbox for stimulus presentation and wanted to verify SOAs externally. It seems digital oscilloscope could also be used for the same purpose as BBTK toolkit (mentioned in the link you have shared). I am looking for ways to how to use digital oscilloscope for the stimulus timing measures. Please let me know, if you have any suggestions on this.

Thank you Atul Gopal, I will check and will update you on this.

Agreed with Huw definitely. The instrumentation and software is called PyGaze or PsychoPy.

A compact, commercial option is http://vpixx.com/products/tools-for-vision-sciences/visual-stimulus-displays/viewpixx/ . It can be directly controlled through Octave/Mtalab (PTB), Python (PsychoPy) or low level C programming. You can synchronize all of your subject I/O to video refresh with microsecond precision and also verify the synchronization by sending a TTL pulse. Check these PTB demos: http://www.vpixx.com/manuals/psychtoolbox/html/Demo14.html and http://www.vpixx.com/manuals/psychtoolbox/html/Demo8.html .

It is not just a synch box. It also includes a time deterministic LCD display as perfect replacement to CRT, A/V controller (that removes many sources of timing uncertainties/latencies in stimulus generation, presentation, and response recording, involved with modern PC with non-real-time processors and OS ) with various integrated digital and analog I/Os.

So, if your concern is just to verify the timing of stimulus generation, using a photo-diode (on screen) assembly to generate a trigger (and perhaps a microphone near the ears, for audio. This too may have an amplifier assembly to generate a trigger at onset of stimuli) is much cheaper option.