You can find the difference between aspects in how the signs work with each other. It can be really hard to solidly tell a difference between a sextile and a trine, since they can bring similar, harmonious effects. However, this does not mean their effects are exactly the same.
While both trines and sextiles are harmonious, the two aspects are very different in what they’re composed of. A trine consists of two signs that are of the same element and polarity while a sextile is composed of two signs that are of different elements, but share the same polarity.
This is important to note because signs that are trine to each other work so naturally and easily to where the native can often not notice the workings of this aspect. This is why people tend to say that trines can either be used for natural talent, or be wasted due to ignorance. The reason why the trine works so naturally is because the two signs involved have the same element, in other words, medium to work with. For example, water signs use emotions as a medium to express themselves, and when two water signs are brought together, they can easily work together because there isn’t an need to change the current medium in hand. When there is no change, there is less room for a misunderstanding between the two signs. However, with no change, there can be a lack of movement and stimulation.
Sextiles however, work harmoniously on different, but compatible mediums due to the same polarity. For example, fire and air signs work well together because both of their mediums (energy and communication) are seen in external environments due to their masculine polarity. Due to the mediums both being external, there is less room for misunderstanding, which makes the aspect harmonious. However, since the mediums involved are different, sextiles do not feel as natural as trines since the native must use two different mediums to express the aspect. Natives with sextiles are often aware of this aspect because it doesn’t feel as natural as a trine and will often consciously express this aspect.
*The only exceptions that can be made to this post are the existence of out-of-sign aspects. Out-of-sign aspects need to be looked at more carefully, since the energy with out-of-sign aspects are not only weaker, but can also be different. You can check the differences in the specific signs, planets, and houses involved.