Firstly you should locate your local Social Security office. If you don't have the office number, search for the address and number on ssa.gov The Social Security Administration's website will provide you with the necessary information you need to find the nearest office.
Next, call the Social Security office in your state to check when your SSI check has been sent out. Inquire about your check and the date it was sent. For the Social Security office to give you an accurate assessment of your case, it will need your name and may need your Social Security number.
If you cannot get through to the Social Security office on the phone you can go down to the office in person. When you get to the office, speak with an employee and inquire about the status of your check.
You can also check the schedule of Social Security Benefit Payments section on ssa.gov. This section of the website will display a calendar showing the dates of when checks will be sent out. According to ssa.gov, if you are a Social Security beneficiary receiving payments prior to May 1997, you will receive your Social Security benefits and SSI payments on the third day of every month. However, SSI payments may be sent out on the first day of the month if the normal date falls on a weekend or holiday. There is also a detailed description of the payment disbursements; if you are a beneficiary after April 1997, your checks will be sent out based on your birth date.