One of the best ways to get an idea about the corporate life that awaits you is to do an internship in a company of your liking. It acclimatises you to the organizational culture and lets you network with important people who can help you kick start your career. However, since the ultimate goal of an internship is to milk some training and learning out of it, steer clear of bad internships displaying the following signs -
You are only doing the 'dirty' work
As an intern, you cannot expect to execute the most crucial tasks for the company. But, if you spend most hours of your internship just fetching coffee, making copies, or shredding junk, then that internship is something you shouldn't be doing. Doing these once in a while is alright, but these should never be your primary job.
You don't have enough to do
If you find yourself at office during ungodly hours, ask yourself if you really are adding any value. A lot of internships just require you to go to office, sit there for 20 hours a week, and return home with nothing but the daunting knowledge that you haven't really gotten anything done. Your time to make a difference is limited; do not waste it doing nothing.
You aren't getting paid
Sure, internships aren't going to pay you in 6 digits and rarely even in 5 digits. But an internship that doesn't pay you anything at all is more likely to reinforce the stereotype of interns as coffee-fetchers and copy makers. Moreover, a paid internship looks more credible on your resume than an unpaid one. The only time you should go for an unpaid internship is if it promises and delivers actual hands-on learning experience.