There is no rule of thumb that has been set out there to be followed when it comes to clothing and fashion in general. Nevertheless, the mainstream commercials will tend to set a trend that most people will accept as the prevailing clothing or fashion of the time. The trend so set will however change depending on the culture or industry in question. It is agreeable that tastes and preference of people vary widely and thus what passes as mainstream clothing and considered the normal way of clothing will pass as alternative clothing to another person.
Should you search the words 'alternative clothing' through any search engine, be sure to have on the top of your list emo and goth clothing. This can lead one to quickly conclude that these are what constitute alternative clothing. This however, is not true if you look at it from a different angle. Why? You might ask. For a goth or emo however, that being their normal way of dressing; they do not consider it alternative. Therefore, for one to be able to conclusively arrive at a reasonable answer to the question of what alternative clothing really is; they need to answer the question, alternative to who? or what?
Fashion is volatile and what is depicted today as the acceptable clothing will be quickly overtaken by another tomorrow. The society has also grown to be more liberal and people accept almost any mode of clothing. This further complicates the issue because one will therefore want to know what alternative really means when the society has adopted the 'anything goes' approach towards clothing. Consequently, what is considered alternative clothing will differ from one person to another and from time to time.
For a lawyer or a banker who needs to be in a suit as required by the profession, a t-shirt and jeans can be considered as alternative clothing. Someone who is into rock music and has adopted the rock music culture will consider the hip hop culture of dressing as alternative. It therefore seems like the whole alternative thing will greatly depend on one's choice, preference, cultural or religious background. Whether or not you are going to settle on goth, emo or punk clothing as the alternative clothing or not, the Indian saree or Spanish gilet, the choice is yours because what is alternative apparel to you is normal or ordinary clothing for another person. If you have to consider what the larger population has accepted and adopted, you can easily acquire an alternative clothing by adopting a unique style of clothing as long as you put into your mind how presentable you are and the functionality of the clothing you have adopted.
Ultimately, you can consider any clothing that is unique from what is being advocated for by the larger population as alternative clothing.