Credible sources, battles or clear Russian troops on the other side of the border. Or, bombs them, or does drone strikes. Or other kinetic attacks.
Threats leading to concessions of territory or political union would not count because it's too hard to evaluate. There has to be at least a shot fired.
Latvia is a NATO member state; they would almost certainly trigger article V which would force all NATO allies to send assistance -- basically spelling doom for any Russian aggression in latvia. The economies of NATO member states dwarf Russia's about 20x so any long-term confrontation would be economically un-maintainable.
One thing I would like to knowm more about is Russian sentiment towards latvia as a nation / territory: it's possible that they feel v. strongly .re this and my western POV is failing to take this into account.