One strategy I've seen organizations do with OFAC/PEP checks is to simply perform these searches more frequently or use some type of real-time screening tool. There are risk intelligence tools on the market that can help screen for both compliance and reputation risk (e.g., Venmonitor through Venminder). As you mentioned, the OFAC website isn't always current and it may take some time for something to show up, so increasing the frequency can be effective.
In addition to onboarding new vendors and regular re-assessments, some organizations are running OFAC checks on a weekly or monthly basis, or whenever they make a payment. One option is to ask your accounting team for a list of vendors who were paid every week or month and run that through an OFAC search.
As far as validating these results, I would say this relates to your other due diligence efforts. For example, have you validated that these vendors have appropriate compliance policies in place, such as BSA/AML risk assessments? Does the vendor have its own third-party risk management policy that includes OFAC/PEP checks on its vendors? Do they have evidence of internal and external audits? Collecting and reviewing these types of evidence can help show auditors and examiners that your organization is being thorough when conducting vendor due diligence, which includes more than an OFAC check. And if you want to be even more thorough, consider consulting with an attorney or compliance expert who can help determine whether your OFAC checks are appropriate for your business.
I hope these suggestions help and I'm interested in learning how other members are performing these searches.