There is a text where Buddha says that the Dharma he teaches is not a secret doctrine so why are there sects which maintain secrecy? These three things are conducted in secret, not openly. What three? Illicit affairs, sorcery, and wrong views - says Buddha. So does this mean that secret teachings of many monks like Yogis I believe are doing one of these three things?
Buddha said that the Dhamma he teaches is not a secret doctrine...By this he meant that the Dhamma is out in the open for everybody to know...It was there even before the times of Siddhartha Gautama. The only difference is Buddha stated it explicitly so that even lay people could understand it. Although the language of the priestly class during the times of Buddha was Sanskrit, Buddha taught in Prakrit/Pali the language of lay people so that everyone could practice it. Buddha's Dhamma is 'Svakkhato' i.e. it is well propounded and well explained by the Buddha. So in that sense it is not a secret doctrine.

And regarding secret teachings, I don't know what exactly you mean by that. But to check if something is Buddha's Dhamma or not, one yardstick would be to check if the teaching helps us to


Refrain from evil doings
Engage in good works
Purify our mind


Also the suttas from the Pali canon are freely available these days on various sites. So the best way to capture these posers is to keep an open, rational and analytical mind and develop our own wisdom through the practice.