Best fluid to see if the valve/seat are sealing well

not water, the molecules are too big, and clump together by surface-tension, sticking to the surfaces like magnets.
Not alcohol, the molecules are so small they will creep by capillary action, thru almost anything.
That leaves gasoline, but you know, don't burn your shop down.

I use alcohol when I want to show a customer how lousy the valve job is that he got done somewhere else. It does what gas does, but is faster.