Licensing is in general a difficult subject and for data even more so. Our goal must be that consumers of OSM data can use our data relying on that a high level of care has been taken to assure that they can actually use the data on the terms of our distribution licence the ODbL 1.0.
The Licence Working Group (LWG) has previously issued guidance on some specific licences that are in common use, but there are a plethora of custom licenses that are used by potential data sources and it is not possible for the LWG to inspect every single one and we have to rely on the OSM community to do at least a rough triage before escalating such issues to us.
Please realize that determining if the licence is compatible is only a small part of the work that has to be done before a source can be used for OSM, particularly when you intend to directly import third party data you will need to lollow the Import Guidelines.
Licences that are not compatible
If a licence is determined to not be compatible you always have the option of asking for special permission for use in OSM from the data source.