How do I figure out if my board is approaching end of life?
Unfortunately, there isn't much way to really determine "wear" in the context of solid-state electronics.

Probably the things that are most likely to fail are the electrolytic capacitors and the connectors.



First, if you're using an ATmega CPU for something that could possibly injure someone CONTACT ATMEL AND TALK ABOUT SAFETY PRECAUTIONS. The ATmega CPU used in most of the arduino models is not rated for use in such situations.

In EVERY datasheet:  


  Atmel products are not intended, authorized, or warranted for use as components in applications intended to support or sustain life.


Now, realistically, this is probably mostly lawyer repellent, but you should still take appropriate precautions.

Really, while there isn't anything on a common arduino board that really wears out except the connectors, why are you trying to save $30 at a potential huge cost? Just buy a new board.

I'd also strongly reccomend you chose a board with a SMT ATmega328P, since that removes the IC socket contacts from the list of concerns. If possible, also remove the pin-headers, and solder wires to the board directly. Try to minimize connectors, since they are frequent points of failure.