Exact meaning of lifecycle of sidecar(prestart)/main task

Trying to utilize nomd 0.11 lifecycle to express the task dependency. task sidecarA (sidecar type, long-run process) and main task B. where task B has to wait for sidecarA fully ready (by service health check?).
My question, when we deploy the job, how we know sidecarA is already ready for task B to be able to start? This is different from the non-sidecar case , which the exit of prestart task means task B can be started. For long run sidecarA, how do we know B can start? I register sidecarA also as a service. Does that mean only if the consul health check passes, B will start? if it is not the case, how do we do that? Thanks.