Received a large payout from an insurance company upon the death of their spouse?
OP = Jessica Fletcher
Why not just ask them?
No legal way to find out anonymously, if that's what you're asking.
If you can't ask or you think the surviving spouse is lying to you - hire a private investigator.
It is inexpensive and quick for a PI to run a financial check on someone. A large payout will show up. It's done electronically and invisibly.
You also might check Probate records. Insurance payouts should show up as part of the deceased's estate.