If you've found a family Bible, you're no doubt ecstatic about the discovery, but you need to look for a few things to determine the Bible entries' accuracy. Just because you found Great-grandpa's birth date in the family Bible doesn't mean it's, well, gospel.
First, always check the date the Bible was published against the dates of births, marriages and deaths. If the Bible was published in, say, 1878, but the entries predate its publication, then you know the information was not recorded at the time of the event, but much later. In that case, the dates of these entries may not be entirely accurate.
Don't assume that the entries were written at the time of the event. Are all the entries done in the same hand and in the same color ink? That may be a clue that they were all written at the same time. While someone in the family may be the designated Bible recorder, when that person dies, are there entries in another person's handwriting? You need to look at the original entries, not a transcription, to tell this.