There may be one situation where you (more likely your spouse) might need to prove you were separated for the entire last half of the year. Even then, you don't need to prove it at the time you file. Proof would only come later is you were questioned by the IRS. Proof may come down to affidavits from people your know (e.g. the friend you moved in with).
As the other reply said, normally your filing choices are Married Filing Separately (MFS) or Married Filing Jointly (MFJ). But, if one of you has a qualifying dependent, and you lived apart from your spouse for the entire last half of the year, that spouse may be able to file as Head of Household. The other spouse (unless he has a different qualifying dependent living with him, for the last half of the year) has to file as MFS.
.If you qualify, Head of household gives you a bigger standard deduction and lower tax rates than MFS, but not MFJ.. The main requirements for Head of Household are: a taxpayer that
1) is single or did not live with their spouse at any time during the last half of the tax year.
2) has at least one closely related dependent (usually a child) that lived with the taxpayer for more than half the year
3) pays over half of the support of the household expenses, including rent / mortgage, utilities, food. As a rule of thumb, this means only the higher income parent can claim HoH and he/she would have to have a dependent to do so.
https://turbotax.intuit.com/tax-tips/family/guide-to-filing-taxes-as-head-of-household/L4Nx6DYu9
