How do you know when a person is preaching upon the sunnah?

[Sundus.Abdul.Sattar]

Assalamualykum warahmatullah,

We live in a time where there are many islamic ideologies, sects, groups etc and there are many events and gatherings held by them. There are gathering where people seem to be preaching upon what is in the sunnah and referring scholars like Sheikh ibn Taymiyyah and Sheikh bin Baaz but yet they are refuted (some are not but they are not upon the sunnah either). Due to the fact that sitting with the ahlul bidah is impermissible , how is one to know if someone who is the speaker of the event is upon the sunnah?

For example, i use to listen to a lecturer named Abu Mussab Wajdi Akkari who owns the website 'one way to paradise' until i heard that he may not be upon the sunnah or that he wasn't a good person to listen to because no one knows who he studied under. He seems to be someone who was upon the sunnah and someone who preaches it so someone who did not deeply research would not know to stay away from him. Al Maghrib institute I know is something people should stay away from, but many defend it saying that the people are from the sunnah because they graduated from Madinah university etc.

In general, i want to know, how do you know specific person is preaching what is correct? How do you know for sure that a masjid or gathering that you attend is really upon the sunnah? How do you know a specific website is giving the correct information about Islam?

Some people easily can sense when something is not right by observing but I know people would not be able to tell. How do i explain to them?

Jazakallahukhair