@PiersMH, Thanks for posting in Q&A. Based on my researching, Get managedDevice API can check if the "lostModeState" is enabled or not.
https://learn.microsoft.com/en-us/graph/api/intune-devices-manageddevice-get?view=graph-rest-beta
Meanwhile, here is a link list the script to enable Lost Mode
Note: Non-Microsoft link, just for the reference.
Hope the above information can help.
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