I'm not in a relationship right now and starting online seemed to be safest.
Hmm... not necessarily. It's more like the risks and precautions to take may be a little different.
One thing that's good for newbies to know is that there are just as many bad apples online as there are in RT. Online, for ex., there are quite a few scammers and predators looking for young and naive newbies to "teach", "mentor" or "train". Usually their "training/ consists of having people do sexual stuff on cam or send pics. Many of them will say things like "You're not a real/true sub/slave, blah blah blah..." to get what they want. So here's your head's up from someon who has heard it all bot as a mod and first hand here
Truth is: finding the right person isn't a whole lot different than vanilla dating: You talk and/or meet with people, look for compatibility, get to know each other
as people and not just as dom/sub or what gets you off, etc. etc.
I've just assumed myself to be a sub, but how can I be sure? Is there a way to be sure?
How people find out or are sure is different for everyone. But one thing to consider is that dom and sub are far from being the only two options. And you don't have to label yourself as anything right away either.
D/s is simply a relationship dynamic. Would you like for your partner to be the one in charge, in control, have authority over areas you have negotiated and agreed on with them? That's what D/s is about.
However, you can very well be into kinky stuff without D/s. You don't have to be in a power exchange dynamic to enjoy kinky activities with another partner. Some people engage in topping and bottoming without any power exchange outside of a scene. Some people are sadists, masochists, etc. without incorporating any D/s in their relationships.
How do you know your actual limits? Is it all trial and error?
Some of my limits I've know forever and have not changed since day one. I don't think I need to try amputation or illegal stuff to know it wouldn't be my cup of tea for ex.
As @df6wen said there are things that pretty much anyone will just think "Hell no" without hesitation whether it's for safety, healthy, ethical or personal reasons.
Can you tie yourself up, properly? Does it still feel good?
Depends what type of bondage you try and how good you are at it (although that part is likely to come with practice.) Tying a simple chest harness on oneself is relatively easy without any outside assistance. Trying more intricate stuff may be more difficult and/not perfectly safe. But for the more simple stuff there are lots of good tutorials all over the web, including youtube so you may want to check it out if that's something that interests you.