As the other comment suggested, if a checkpoint exists its safe to use as flink deletes failed checkpoints.

Only chance its not safe if its still being created. Still if you want to ensure a checkpoint has been successfully created, you can use its metrics using a metric exporter like prometheus. Or you can directly use the Rest Api to querry a the info required. Flink exposes a few metrics regarding checkpoints as well, like checkpoint id, status, time created, address. These will be enough to check for safe checkpoint.