Most of our safety valve vent directly into a flare header making it difficult to determine if one has lifted. One way is to put a bursting disc  on the discharge side of the safety valve. This solution is however expensive to retrofit on an existing plant.
If the medium is at a higher temperature than atmospheric, then a thermocouple (temperature sensing element) at the outlet may serve your purpose.
I have an internal requirement to investigate everytime a safety valve lifts. With over 150 valves which don't have associated instruments that measure / record upstream pressure, and very limited funds I am trying to find a simple, cheap method of determining when one lifts
Check with the major PSV vendors.  I believe Consolidated (and likely others) have PSV lift indicators but I'm not sure what retrofits involve.  I have seen them talked about in literature but can't remember the vendor.

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