There are a couple of ways to value these accounts and that really depends on what type of plan it is. A defined contribution plan is much easier to value and therefore easier to divide within the divorce. The defined contribution plan has a certain value as of a certain date, which can be divided as either a percentage or a dollar amount to the other spouse.
A defined benefit plan is valued as a future stream of payment based on a benefit formula. Most defined benefit plans are pension plans, which will not pay a lump sum amount and will only pay a spouse on a monthly basis for a lifetime starting around retirement age. The actual value is usually unknown until that benefit begins on that later date.