If FRE don't apply (1101), then what is the standard?

In certain hearings, the rules of evidence do not apply. See FRE 1101(d) (preliminary questions, grand jury, sentencing, etc.). If the rules don't apply in the hearing, then what is the standard for letting in evidence? In an unpublished Fourth Circuit case, the court cites the federal sentencing guideline which states the requirement that "the information has sufficient indicia of reliability to support its probable accuracy." Of course this is a specific rule that refers to drug quantities, but it gives an idea of what type of standard the judge should consider when hearing evidence at a non-trial hearing.

18-4484

United States v. Bagley, 774 Fed.Appx. 161

Here is an interesting paper as well that goes a little deeper into the rules of evidence and suppression hearings.

Evidence Professor Wittlin has also explained that just because the rules of evidence do not apply, doesn't mean other constitutional protections do not.

There is probably a more specific answer out there...I will keep searching.