The treatment of growth in the IRA as marital or separate property is a matter of state law, so you should always consult the law in your state.
If you have IRA statements as of the date of the marriage, the separate property portion of an IRA typically is held to be the value of the IRA at the date of the marriage.
That is, typically, contributions that are made to the IRA during the marriage and all growth that occurs in the IRA during the marriage are treated as marital property.
In the unlikely event that no contributions are made to the IRA after the marriage, and the IRA is treated by the parties as separate property, then growth in the IRA during the marriage may be treated as separate property.
If you do not have a statement of the value of the IRA on the date of the marriage, you may estimate the marital property portion as the current value of the IRA multiplied by a coverture fraction. The coverture fraction would be calculated as: Number of days during the marriage / Number of days the IRA has been in existence. The separate property amount would then be: current value – marital property portion.