how to know that person is really love u or just pretending to love u

Many ways

Hindsight.

There are no surefire, 100% guaranteed, external shortcuts you can pre-apply to any and all random strangers to absolutely determine their internal emotional state, thoughts, and/or feelings, setting them in concrete.

At best you can come up with things that mitigate the risks of them pretending.

Not possible.

Love is felt only within yourself.

Living people exist in the Now.
Perhaps you should consider how you feel in the Now, while you are with them.

It might help if you can define what love means to you.
Make a list of feelings you experience while you are with the person and while you are away from them.

If you understand what you love and don't love, you can determine if the love they show fills your needs.

Also try to realize if you suspect their honesty, you are probably right.

Communication is key to relationships. Perhaps you should speak with the person about your concerns. Its entirely possible you each define love in different ways.
They may feel love for you but in a different way than you feel love for them.
Both are valid however they may or may not align with each other.
Only honest communication will reveal the truth.

Meet in person and have Conversations then Kiss

Noticed him/her in angry mood

Cut off contact and see what happens. If they love you, they will check on you. pay attention to where you are on their priority list. When are they spending time with you? When it's only convenient for them? To fulfill their needs only? Do you have similar goals? Does the person ever back out on plans? How often does this happen? What is the reason/excuse? Do you have something they want? Money? Luxury lifestyle? If you suspect it's your lifestyle/money the person wants, hold off on the spoiling to see if he/she sticks around.