A rather difficult question we need to ask is exactly what is heresy? The Bible tells us to all be of the same mind, preaching no other doctrine, and to avoid division. We are even told to remain separate from heretics. But what is heresy?
It is a tragic fact that, because we are weak in our flesh, we will unfortunately fail to be perfectly united. There are several things we might reasonably have differences of opinion. If we call all differences of opinion heresy, then we must conclude that we are all heretics, and there is a maximum of one Christian in this world. So, once again, what is heresy?
Secular dictionaries provide minimal aid here. Definitions include "adherence to a religious opinion contrary to church dogma", or "dissent or deviation from a dominant theory, opinion, or practice". But neither of these are particularly helpful. If we go by the first definition, which Church's dogmas take precedence? Heresy can't be that which is contrary to Church dogma, because there are too many Churches with too many distinct dogmas. Not to mention that various Churches change their dogmas over time, even to the point where their own founders would ultimately be considered heretics today. Likewise, the dominant theory, opinion, or practice may change over time.
But one thing that doesn't change over time is the Bible. The Bible has been solid for over 1,900 years, and it just happens to be the very word of God. We can say certain beliefs are heretical not because a Church told us, and not because a majority of nominal Christians tell us, but because the Bible tells us. That which can be categorically refuted by Scripture is a heresy. That which can at least be reasonably understood from Scripture is not heresy.
Things like Theistic Evolution are heresy. Theistic Evolution is a brand new belief that was introduced during the 18th-21st centuries in an effort to fit with the dominant secular view of origins. It cannot be gathered from Scripture, it cannot be forced to fit Scripture, and it even causes division within the Church. Therefore, we can say that Theistic Evolution is an absolute heresy. Other views are not so clear cut. For example, should communion be taken from a single cup, or should multiple cups be used? Scripture does not give us a clear answer. Some Churches may use one cup, others may use several. Both are valid, and neither is worth dividing the body over. Thus, neither side of the disagreement are heretical.
Ultimately, avoiding heresy comes down to a faithful exegesis of Scripture. Stick to what Scripture says, and you will never be steered in the wrong direction. The errors of others should be assessed based on the severity of the error, most notably the effect in both the individual's life, and the effect it has on the Church as a whole. The Bible tells us to twice admonish a divisive heretic, and if they still refuse, excommunicate them (Titus 3:10). But we must do this with wisdom, lest the Church become so divided, we dishonor our Lord.