What is the criteria for proof that you used a second home as primary residence?
If the answer you are seeking is regarding the IRS guidelines for criteria of a second home being a primary residence then your considerations should be as follows:
You must establish residency in the State where your second home is located, e.g., automobile and voter's registration, driver's license, filing for homestead, bills and banking statements, and file your tax returns using this address.
You must own and reside in the property for 24 months (or 730 days) out of the last five years. However, that 24 months may be, but does not have to be, continuous.
In other words, in 2004 you resided in the property for four months, in 2006 you may have been in residence three months, and in 2007 you were in residence for 12 months - equaling 19 months. You still need to reside continuously in the property for an additional five months to qualify as a primary residence. Short term absences, such as a vacation, do not invalidate the residency, but a one year sabbatical would.
To use the tax break given to married couples of $500,000 on the sale of a primary residence and $250,000 per qualified single, then you may not have used this exemption on the sale of a previous primary residence within 24 months of the sale of the second home that you have established as your "new" primary residence. You may use this exemption over and over, but again, not more than once during the two years prior to the current sale.
Further, you do not need to own the second home for the full five years prior to the sale as many believe.
There are exceptions to this rule in the case of "unforeseen circumstances" and you should check with your tax advisor if you feel you have established a previously used second home as your primary residence and you are forced to sell prior to the "use" periods as described.