How do I know if I should select the general rule on my distribution from my 401k and I am retired and live off of this money I withdraw?

[Bsch4477]

You pay tax on your 401-k withdrawals at your regular tax rate. The general rule applies to pensions and annuities. 

[dmertz]

Unless the 401(k) plan has lost their records of any after-tax money in your 401(k), the Form 1099-R will show the actual taxable amount in box 2a and box 2b Taxable amount not determined will not be marked.  In addition to the General Rule not being applicable to distributions from a 401(k), the Simplified Method should not be used unless the distributions are periodic distributions and either box 2b Taxable amount is marked or you wish to dispute the taxable amount shown in box 2a and you will provide your evidence to the IRS as to why the amount in box 2a is wrong.