help with executing instruction every i-th run of loop

I am running a very long loop and would like to save intermediate 
results in case of a system or program crash. Here is the skeleton of 
what my code would be:

for (i in 1:zillion)
   {
    results[[i]]<-do.something.function()
    if (logical.test(i)) {save(results, "results.tmp")}
   }

logical.test would test to see if i/1000 has no remainder. What R 
function would test that?