In interview some companies don't require formal clothes,how do you know that??

1. Professional Dress: If you're interviewing for a career in a professional environment, such as business, finance, or anything in a traditional office, it's important to look professional so your employers can see you fitting in with the people in their company. If you show up dressed too casually for the part, you will stick out like a sore thumb and will look like you're sloppy or don't know how to follow directions.

2. For Casual Business Dress Casual : Business casual is defined as being in between professional and casual dress, and it's important to know which jobs require business casual dress. Jobs that include working in a site that requires special equipment or that is dirty tend to accept business casual dress. They jobs include work in a laboratory, warehouses, construction site, etc.

3. Dress for a casual environment. Some work environments, such as those at a startup, have more casual dress. You can check the environment by looking at the type of company, or even checking the company's website. If you wear a full suit to an interview at a casual company, you will look stodgy and too stuffy to fit in.

It is important in the interview the first impression I think is better to be fit clothes. and after start I think no one will be interested if he come or not in suitable clothes. especially If his job location outside the company building.

The best way is to ask the person who has called you for the interview, whether the dress code is formal or casual.

In case you're not sure, then be safe. A formal dress-pants, shirt, shoes and tie, should do the trick.

I would call the company and tell them that I intended to come to the interview properly "suited" unless they expected anything different. This then lets them know that I was prepared to attend suitably dressed but gives them the option of informing me if this wasn't a requirement and thus ensures there is no embarrasment on either side.