How can I be sure I am lonely?

Normally an obvious answer would be you have no friend to talk to. That's not for sure your lonely though. You are lonely when no one has your back when you in trouble or in need of assistants. You are lonely when you have nobody to tell all your feelings. Sure you can talk to a therapist or even tell a random stranger. Guess what though they not even care about you. They won't remember what you said minutes after you're gone. You are lonely when no one sees your ideas the same way you do because they are not with. That there is my opinion on being truly alone. I know I may have not covered all that there is to loneliness but you gotta start thinking what you think loneliness is.

When you're surrounded by people but still feel you are alone. In this case you can say that you are lonely.

One feeling that is typically characterized by loneliness is a sense of emptiness, or feeling as if something is missing, whether someone or something is missing from your life or not.

Do you feel that the quality of your relationships (family, friends, acquaintances) is lacking or could be way better? If you said yes to that, the answer is probably yes, you are probably lonely.

If you do not have many close friends and are not close with your family members, you may be considered lonely.

I would know I'm lonely if I feel like I have no one who care about me and no one to talk to. If I feel like I have no one to share things with when I'm happy or sad I would feel alone

if you have no one around you, but you should have people be with you to make you not lonely. People who care about you and love you.

I don't think there's any "being sure". If you've had to even ask yourself this question, you probably are. And that's okay. I've dealt with this problem before and I still do. I searched everywhere for friends and no one really wanted me around or to keep a conversation with me. I put myself out there but it wasn't enough. I got a job, moved, changed areas. Iit took me about 8 months but I'm finally surrounded by friends. Not many, about 3 or 4, but it's way better than none. Don't stop trying. Never give up. Someone somewhere is also needing a friend. You just need to find them.

if the feeling ensues nonstop... but we are never truly alone. strike up a conversation with an old friend and talk for a while- or message a bunch of people all at once. it usually works for me :)

Y'know, you can feel loneliness without actually being lonely. If you are feeling loneliness then those feelings are completely valid - but loneliness doesn't mean you don't have anyone to talk to, where as lonely does. So if you are experiencing loneliness then recognise that you have people around you to talk to who maybe feel like that too sometimes, and so you talking to them (ABOUT ANYTHING) will probably help them too. And if you're lonely, then I encourage you to push yourself to explore new interactions and ways of getting to know people, for your own good, and for theirs too 'cause they'll get to meet you!