To ensure you have a clean lower ready to be made into a pistol, the manufacturer of the lower would need to provide you with a letter describing the lower as 'virgin' i.e. neither a pistol nor rifle. Then you have to go to the dealer you purchased the lower from and ensure the receiver was not recorded as a "long-gun" on your form 4473. If you build a pistol on a lower recorded as a long-gun (rifle), you have made a SBR.
Generally, the only way someone would get busted by not following the steps above ^^^ would be if your "pistol" were stolen and later recovered by the cops and through the process of tracing the serial number from manufacturer to dealer would the cops discover an illegal SBR if "long-gun" appears on your 4473. Shortly thereafter you'll have the local SWAT & ATF kicking your door in. You'll be charged with unlawful manufacture of a firearm, unlawful possession of a SBR and falsifying a police report (you claimed your "pistol" was stolen when in fact it was your rifle).
Something else, you can also use a 6 position buffer tube on your pistol (stock removed) but ideally you sell the stock or give it away. As others have said, just having all the parts to build a SBR is the same as being in possession of a SBR which is a violation of the NFA. Here is a copy of an ATF letter I found via AR15.com
