Some friendships survive distance because neither person requires constant proof.

Some friendships require constant maintenance because they run on reassurance. Others do not, because both people trust the thing exists whether or not it has been recently demonstrated. That trust is what lets a friendship survive years and continents, and it usually comes from a history solid enough that neither person needs it restated.

If a friendship goes quiet, resist reading silence as a message before you check. And where you are the one who needs proof, consider asking directly rather than testing. Directness costs less than the testing does.