Examples from Texas community property law (these are from my notes from about ten years ago, so I don't know whether the law has changed).
1. Gain on sale of separate property personalty during the marriage -- such as an automobile acquired by one spouse prior to the marriage -- would generally be the separate income of the spouse who owned the separate property, assuming that the gain is spontaneous and independent of assistance from the community.
2. Gain on a sale of separate property real estate during marriage would generally be separate income of the spouse who owned the property. Again, this assumes that the gain is spontaneous and independent of assistance from the community.
3. Funds or other property acquired by one spouse to compensate for personal injury to that spouse during the marriage would be the separate income of that spouse. However, compensation for loss of earning capacity during the marriage would be community income.
4. Oil and gas royalty income or "bonus" from separate property mineral interests during the marriage would be separate income of the spouse who owns the mineral interests. (But, rent income and delay rental income from the separate property mineral interests would be community income.)
5. Funds or other property acquired by only one spouse during the marriage by inter vivos gift, or by will, or by intestacy succession would be the separate income of that spouse.
Note: In Texas, you have to consider that there is a legal presumption that anything acquired by either spouse during the marriage is community income. However, that's just a presumption -- that is, a rebuttable presumption.