That's the whole point of health monitoring in an engine, a.k.a. man-rating the rocket.
Do you think the engine is chugging along nicely one moment and the next one it just goes BOOM?

I asked (as "Anonymous Coward"), several comments ago to point out one explosive liquid engine failure. If the troubled N-1 from over 40 years ago is the only example then I rest my case.

For comparison, I'll point out two recent rocket failures:

1) Sea Launch Zenit where debris caused the engine turbopump to fail, the rocket lost thrust, fell back to the pad immediately after liftoff and blew up. It was loss of thrust, not explosive engine failure that doomed the rocket.

2) Soyuz Foton-M launch. Immediately after liftoff, debris in one of the booster engines caused problems that were correctly picked up by the flight computer and the vehicle simply shut down all the engines (as part of the flight termination system). Still intact, it fell down to the pad resulting in a major explosion. Again, loss of thrust doomed the rocket, not explosive engine failure.

And they didn't have a test stand because they didn't have enough funding to conduct a full-fledged test program. Again, what is the point of this? An engine blew up during N-1 flight testing. The 30 engines were never fired in close proximity to one another and acoustic loads were too high, not really surprising. If the had done a static test, they'd probably caught that on the ground. You don't find that out in operational flights, you find that in test flights. Saturn V F-1 engines blew up during ground testing as well. It's called development phase.

N-1 cannot be really brought into the comparison. You might as well invoke the early Atlas boosters then. The time schedule pressure N-1 had, lack of funding causing very few crucial tests performed just *had* to doom it. N-1 never had a first stage static test performed! It wasn't a case of operational vehicle having engine failures (which is what we're discussing here), it was a test vehicle that was plagued by design problems. It can be compared to Apollo 6 POGO which if manned would also have led to an abort, but it didn't - it was a test flight. Operational flights cannot be compared to test flights.