Its use is not encouraged by the people in charge of Ogg. It seems to have been mostly supplanted by MKV and other non-Ogg formats, while the true Ogg believers restrict themselves to Ogg-friendly formats including Ogg Skeleton.
See Ogg#Identification for identification of Ogg in general.
Based on FFmpeg source code and other evidence, an OGM file is an Ogg file containing at least one "OGM stream". The known OGM stream types all begin with a signature consisting of an
0x01 byte, followed by one of the following ASCII strings: "
Direct Show Samples embedded in Ogg".
The data for the first stream in an Ogg file normally begins at offset 28, so an OGM signature might be found there. But sometimes the first stream is a non-OGM stream, most likely Vorbis, so an OGM file could superficially look like an Ogg Vorbis file.