But if you are not preparing the personal returns how would you know that this is rental property from the K1?

If the K-1 is done properly, it will say that.

:+1:

Use the 199A Worksheet by Activity to minus out rentals that you know are not QBI.  While attachments to the K-1 will identify it as a rental activity, some rentals will be QBI and some will not.  If it isn't QBI, no need to flow it through to the partners saying that it is QBI.

@IRonMaN

'Tax Cuts and Jobs Act, Provision 11011 Section 199A - Qualified Business Income Deduction FAQs | Internal Revenue Service'

"S corporations and partnerships are generally not taxpayers and cannot take the deduction
themselves. However, all S corporations and partnerships report each shareholder’s or partner’s
share of QBI, W-2 wages, UBIA of qualified property, qualified REIT dividends and qualified PTP
income on Schedule K-1 so the "shareholders or partners may determine their deduction"."

The K-1 'done properly' is Incorrect.
It's the K-1 recipient that decides whether or not QBID applies at their level.

Their share of QBI could still be zero.