You started with a pretty general question and then wanted to apply answers to your systems without providing details. Maybe if you could provide information on the types of systems you're dealing with you could get a better answer.
If you are dealing with two sources that are frequency and voltage stable, and in phase, then a sync check by itself is probably ok. If you are paralleling a generator to a stable and reliable source, then an automatic synchronizer may not be needed. If you're trying to sync a smaller machine to an active bus, then likely you will also need the capabilities of a control that can change the speed and voltage of the oncoming machine to make it parallel properly.
But based on the info you provided so far, you're really only going to get general answers.
If you want to get some more info on the topic, look at chapter 7 of the attached.
MikeL