To my knowledge we don't. The UN not only has a panel of experts but has, I understand, a separate panel of experts for each different sanctions regime, and that is staffed with people who know very clearly what's actually happening on the ground in that country, who in fact is operating as opposed to who you're being told is operating, who the real movers and shakers are, and so on.
In the European Union, one problem is that the EU's procedures are not very transparent when it comes to sanctions, so we are never told who it is that does this. My guess is that the Council of Ministers, which has responsibility for imposing sanctions, does not have separate teams of experts going out and gathering evidence for each sanctions regime. I think what happens is that they rely on each member state's own domestic evidence-gathering capabilities, and each member state then comes to the EU with a list of people and with reasons supporting why they propose that those particular people should be subject to sanctions. At the EU level itself, though, there is no equivalent to the panel of experts.