A very wide variety of traits, but most of the key diagnostic criteria according to the DSM-5 are related to social difficulties. Talking too much and not realizing, talking too little and not realizing, not understanding the flow of conversation to the point where you don't know when it's okay to speak, struggling with small talk, constantly feeling like people misunderstand your words, speaking very bluntly without understanding the impact of your words, the list goes on...

Individuals with ASD might also tend to stim (repetitive motions intended to stimulate the nervous system, things like rocking in place, or bouncing your leg...) more than allistics and in ways that are less socially acceptable. Repetitive thought cycles, a tendency toward routines, and discomfort, when they are broken, are also common. I could keep writing for a very long time because the way neurodivergence expresses itself across different parties can look very different. I haven't even touched on sensory issues, or digestive issues... I think your own independent research on this matter is most important; if you feel like you relate a lot to the experiences of other autistic people and you relate to a decent portion (not all) of the traits commonly associated with ASD, there is a decent possibility you're autistic. As an aside, try not to compare yourself too strongly with the details of any particular autistic individual's experience. It's highly unlikely you will relate to them on every level. Lower support needs autistic people can often learn to mask their autistic behaviors without even realizing it, and just because you don't immediately relate to a particular behavior doesn't mean one of you isn't masking it. Some people relate to all the social deficits but don't overtly do things like stim. It's all relative.

As for telling if it's just introversion, it can be difficult to tell when you're examining yourself. Do you often struggle to find things to say to the people around you? Do you feel like you have to make a conscious effort to navigate through basic conversations that other people seem to navigate with ease? Has anyone ever been put off by you, and you couldn't understand why? Have you ever been told you're blunt or hurt someone's feelings and been surprised by it? Conversely, do you consciously make yourself as small as possible in social settings and have trouble saying no or expressing differences to the people around you because you don't want to upset them? Ask yourself if you really just have a small social battery or if socialization for you is a barrier that expends far more energy for you than it seems to expend for the others around you. I used to think my social anxiety was the cause of all my social woes, and a big factor in realizing I was autistic was coming to understand I still struggled with every aspect of socialization even when I wasn't shy. I write scripts in my head for calls, and have to brace myself and think of how to greet people before we meet. I am just constantly overthinking the minutiae of social interaction because it does not come intuitively to me.

As for a reason to seek a diagnosis as an adult... in my opinion, no. Assessments at this point in time are expensive and incredibly unreliable. There's too much to say on that topic to really do it justice, but it's a difficult journey pursuing a diagnosis, and at the very end of it, you'll probably still be uncertain. Self-diagnosis is perfectly valid, you as a person are the best at understanding and assessing your own experiences, and until formal assessments become more reliable I think you have far more authority with some research in diagnosing yourself than any other schmuck can. That said, the accommodations you may be able to receive from your employer might be worth the fight, but disclosing your disability can also have a negative impact. You'll have to ask yourself this question and weigh the pros and cons.

Autism has many ways of manifesting itself. Its like a grab bag everyone gets a different mix of divergencies. Personally I think most people have a Neuro diverse trait or two and we only get called autistic when we have so much can't hide them.

I'd say social awkwardness is a common thread. I'm not introverted at all, I don't notice if people want me to shut up.

Getting more than "yes you are autistic" can be pretty involved and you might have to see a specialist for a while but a proper diagnosis figures out what divergencies you are working which helps you come up with ways to cope with them in the allistic world. Having labels and definitions can put things in context and help you understand why human interactions went the way they did in the past and allow you to prevent undesirable patterns from repeating. Saying "I'm autistic in X way and so I need you to Y" goes a long way.

I'd also like for someone to answer, especially question 3.

3. Validation primarily, but with a diagnosis it is possible to force your employer to provide some accommodations. For me, I got diagnosed last year and it removes so much uncertainty.