How to tell if someone is in fact an expert:
Look up their citation index in their field.
https://subjectguides.uwaterloo.ca/calculate-academic-footprint/
..if they have a low or negligible citation index, ignore them.
You can also see their informal rating on https://www.researchgate.net
Another way is to see how many journal articles they've published.
If they've never published a journal article OR it is in a dodgy journal, ignore them.
Dodgy journals are those published for profit which do not do proper peer review.
https://predatoryjournals.com/journals/
