1992 Honda Accord, granddaughter reports that headlights blinked on & off on her way to work this morning... where do I start to find the cause?

It sounds like your problem is the Headlight relay ,under the bonnet there is a block of relays so look in the book for fuses and relays and it should tell you which one is the headlight relay remove it and obtain another one and replace it {take it as a sample}as its opens when getting to hot and then comes back on when cools down.
It may also be a short in the head light wire ,,touching and shorting on the body or a loose wire around the head light assy OR the relay could be loose fitting make sure it preesed in the holder tight.