When should you trust "expert opinion"? Given that we cannot possibly critically examine every claim ever made, here is a three step heuristic (mental shortcut) that will help you to provisionally accept or reject a claim, while admitting that you have not critically examined the issue. Realize that by phrasing your beliefs and views in this way will keep you more open to following the evidence and less prone to the many biases and emotions that will keep you from the truth.
Step 1: Look at the claim itself. Is it a claim of faith? In which case, "expert" status is irrelevant unless you share that faith (e.g., 100% of fairyologist say that fairies exist). The Pope says that transubstantiation (literally turning wafers and wine into Jesus’s body and blood, respectively) is true, and he is certainly an expert in Catholic dogma, however; it is a claim of faith. If it is a faith-based claim and you share the faith, or if it is a scientific claim (empirical, testable, falsifyable), then proceed. It is important to note that the veracity of philosophical claims and claims of faith can only be seen as “true” within the particular faith or philosophical world view. What is the level of consensus among other experts? Generally speaking, the greater the consensus, the more confidence you can have in the claim.
Step 2: Look at the "expert." Is this person really an expert? Expert status could be a result of factors, and is very subjective. Does this person have any clear religious, political, or financial motivations for making the claim?
Step 3: Look at the relationship between the claim and the "expert." Is the claim being made by the expert in the expert's field of expertise? For example, an expert in medicine is not automatically and expert in cosmology. Is a scientific claim being made by someone with no expertise in the scientific method?
Keep in mind that this is a heuristic only, a way to provisionally accept or reject claims made by reported experts. Depending how important the claim is to you, combined with your level of provisional trust in the claim, you can decide if this is a claim that you would want to invest the time to critically examine, or be satisfied with your provisional analysis.