The most foolproof way to verify the results would be to have the client return all the 100 million values back to the server. This may be a bit much though, so alternatively, after submission, send a random selection of numbers in the range back to the client, which will have to return the prime summands* for those. Possibly with a time limit to prevent it from doing the calculation for only those numbers. So it probably also needs to be a fairly large selection.

*I had to look up that word