My understanding is that a Jag mechanic can tell easily... apparently it
has a distinctive rattle for a moment as the engine is started,
long before it gets serious enough to fail.
Or, assuming it WILL go sooner or later, why not just have it upgraded
now, as insurance, and quit worrying?
I think the price is in the $1200 range in the US, mostly labor.

There is a definite rattle, when either tensioner or chain is going. -
usually easily recognised on start up or by blipping the throttle.
Failure of either can trash the engine
I assume its the same as the old jags, XJ6, an engine out job, well you can
do it with engine in but it takes longer.
Any jag specialist will give you a price.