How can I find out whether or not it's permissible to use a you tube video in a church service?
There are two sides to this question:
1. Does the owner grant permission to use it in the church service?
Before using a video in a church service, message the owner. Usually they will be happy to let you use it, and on occasion they will direct you to a higher quality download without ads. In general, YouTube is free to play.
2. Is the church okay with the ads that are likely to come along with it?
Youtubers make their money off of advertising, and they have no control over the ads that are likely to be presented. I recall a time when a children's church program (tee-totalling denomination) started with a beer advertisement because that what was shown with the video we had selected.
If you are live-streaming or recording services for later playback on youtube, you have to pay extra attention to permissions on copyrighted material. Your CCLI license does not cover streaming, but the CSLP license does. Hillsong blocks re-streaming of their content, as do major networks (nbc, etc).
It depends on how the video is licensed. There are two main licenses used on Youtube. The Standard Youtube Licence and the Creative Commons license.
The Creative Commons license gives free reign to use the video for whatever you want, so long as you maintain attribution of the author of the content.
Youtube standard license is a bit more tricky. Section 5.B of the Youtube terms of service reads (in part) "Content is provided to you AS IS. You may access Content for your information and personal use solely as intended through the provided functionality of the Service and as permitted under these Terms of Service. " It comes down to the meaning of "personal use" and if that applies to the playback during the church service. I'm not a lawyer, but I would tend to think that public display probably isn't personal use, so it would probably be a violation of the license.
As far as restreaming that pojo-guy mentioned in his answer, any CC licensed videos would be fine to re-stream as well as long as the attribution is intact for the work being used. Youtube doesn't use a SA (share alike) CC license, so it is not necessary for your stream to also be CC licensed.