Well, this is a really intriguing question and a really interesting one to answer.

Liking is something which develops over a period of time when you start spending time with someone. It depends alot on the way you take things, how you behave, and how much understanding you turn out to be.

There can be some sure shot signs which reflects the intensity of someone liking you-

The kind of eye contact they make, and the interest they take in your talks.
You seem to always top their priority list. How much hard it may be, they try and find out time for you. Always available at your convenience.
They take care of your minute things. (Eg: reminding you to take medicines, wake you up in the morning for your important stuff, helping you in your work, you timely food and sleeping habit, etc).
They always stand like a strong rock backing you so that you might not get into trouble.
They will help you grow as an individual and help enhance your personality.
They might sometimes irritate you by their over caring attitude. But indeed they are helpless. They are bound to be there and care for you.
Your small achievements give them immense happiness and they try to celebrate it with you.
They might sometimes surprise you with the things you like.
These can a few signs which might help you identify who secretly likes you but then “All that glitters is not gold”. So be little wise and take time before choosing people around you.