Even a seasoned research scientist will be qualified to directly evaluate experimental data only in a small subset of all areas. Fortunately, the peer review system allows scientists to make informed estimations regarding the research that falls outside of their area of expertise.
The best case scenario in science is that the experimental data is so overwhelming in support of one theory, that even those previously opposed cannot now justify opposition; the proper conclusion is totally obvious and consensus in the scientific community is ~100%. That doesn't mean that the theory is correct, but it does mean that anyone other than an expert possessing new experimental data or rare insight would probably be irrational or unable to doubt the theory supported.
Typically though, uncertainty centers around the questions that have not been firmly settled. In such a case, it is generally not appropriate even for an expert in the area to draw firm conclusions. The simplest heuristic in this case, is to assign a probability to each theory, where each probability is biased by the level of consensus in the scientific community, and keeping in mind that possibly none of the theories are correct. The more expertise you have in the area, the more you can use your own judgment to modulate those probabilities.
Often though, an issue is too new or fringe to have attracted the critique of the scientific community, or that critique has not reached the public. In this case, there are a few questions that should be asked by non-experts (keeping in mind that this list is non-comprehensive).
How much research is there in favor of the claim? does that research stand in opposition to or in the face of a large body of other research? Does it seem likely that, if the claim were true, it would have gone unknown until now?
Who is promoting the claim, through which forum, and for what potential motives? (If they're not scientists, or it's not published in a scientific journal, be suspicious. Also be suspicious of non scientists or scientist non-experts referencing obscure research in which they were not involved.)
Does the claim offer an easy solution to any of life's problems, and is it monetized by the people who back the claim.
Typically, the best sign that a claim is legit, is when the experimental evidence in favor continues to mount, originating from multiple independent laboratories, along with the approval of the scientific community.