Calculate gain on sale of interest in a partnership

If the partnership's assets consist mostly of land, there may not be much in the way of 751 gain.

Do you have a copy of the partnership or operating agreement? Either under the agreement or state law, usually partners have a right to obtain copies of the complete partnership tax returns, which might help. Do you know the history of your client's ownership? Did the trust acquire the partnership interest due to the death of a partner? If so, was there a Form 706 filed that would show the basis as of the date of the partner's death? Did the partnership make an election under Sec. 754? Have there been disclosures of Sec. 743 step up and depreciation on the most recent K-1s?

If the trust was an original partner, the tax capital account would be a reasonable way to estimate basis. 

Do you have a copy of the sale agreement? Could it have been effective dated back to the beginning of the year? Did it state anything about the partnership's reporting obligations?

The partnership is obligated to provide the information about Sec 751 gain on the sale of a partnership interest. If you can't get it, you could consider reporting on Form 8275 that the partnership refused to provide information about any Sec 751 gain, but that the partner believes there was none due to the nature of the partnership's assets (if that is indeed the case).

Was Form 8308 filed? The selling partner should have received a copy - Partnerships file Form 8308 to report the sale or exchange by a partner of all or part of a partnership interest where any money or other property received in exchange for the interest is attributable to unrealized receivables or inventory items (that is, where there has been a section 751(a) exchange).

If no Form 8308 was filed, that could indicate there is no Sec. 751 gain, or it could indicate the partnership is not aware of the obligation to file Form 8308.