I find it vexing that some people can't seem quantify trust simply because there's only one word for it. Of course there's no continuum or universal scale, but there are at least "ballparks" that indicate thresholds of trust; trusting someone to watch your dog while you're on vacation and trusting someone with your power of attorney are clearly different. Most people have some implicit understanding to a degree, but we still only have the one word, "trust," making it difficult to communicate. It's important to be very aware of these thresholds - despite the fact they lack words to represent them - so as to mitigate empowering someone undeserving. No one's perfect, but you can learn something from almost anybody; knowing where someone stands trust-wise ensures you ask the right things of the right people.
Trust in Sanity -
I suppose you could also call this "Trust in Basic Human Decency." This is the basic litmus test probably applied to every single person you've chatted with online or considered playing with. At this point if you're transitioning from vanilla sex, you might let a guy tie you up and fuck you but maybe not spank you or otherwise push you. Essentially, it's simply trusting that he'll keep his word and not overstep any terms you agree to. I usually equate this with trusting that a guy is also a safe, responsible player, but some people may not have the naive outlook I do and instead approach them separately.
Trust in Competence -
Everyone has different levels of experience, and it's important to be aware of both your own and your play partner's. Unfortunately, not everyone is aware of their level of experience and will sometimes overstate it, even claiming to be into things they've never even experienced. More often than not it's the guys who are mostly interested in fucking since they're just trying to bait you however they can to get in a quick screw. At this point you should be able to have scenes with your limits being pushed, but in a directed way; maybe increasing your flogging tolerance with a competent guy who doesn't happen to be as aggressive as you'd like with other activities.
Trust in Chemistry -
When you feel compelled to continue chatting with a guy to play, there's a perceived chemistry that may or may not exist in reality. Now you've either discussed enough or played enough that you both have a pretty extensive understanding of each other's interests, and there's significant overlap - whether it's just a myriad of interests in common or the fact you both like a scene to have the same atmosphere. Someone at this level would likely be able to write out a scene play-by-play that is notably similar to your fantasies or goals without you giving further input. Because of the similarities, this is when you might stop trying to limit activities in a scene. Instead of setting up a flogging scene you might just go to the guy's house just expecting to be dominated, but requesting a few things knowing that it is only a request and not a condition.
Absolute Trust -
To sum it up: negotiations are over now. This is when a D/s dynamic is really solidified. You know the guy can reliably give you what you need - not just want - so making requests is no longer necessary. Requests might still arise sporadically, but more likely in casual discussions between scenes rather than as specific reasons for wanting to have a scene. The sub will likely still retain some hard limits, but at this point the goal is to give the Dom as much freedom as possible. It's common to hear "The sub is always the one in control," but it's at this level of trust where that line is blurred almost unrecognizably.
It's paramount keep in mind that things can easily look good on paper; if they didn't, it's unlikely you would continue discussing a scene with a given person. Each Dom is going to have an expectation that a sub trust him to a certain degree, and some might even require a sub immediately trust him absolutely from the get-go. Look at what the Dom is expecting from you and compare it to where he falls in how you trust him. If you're feeling uneasy about meeting a guy, there's a fair chance that it's because you're being asked to trust more than you're comfortable with. In that case, try and steer the discussion towards a scene which more closely reflects your level of trust, with whatever restrictions in place will make you comfortable.
Having lived in a small-ish city before, I know that as a sub it's easy to feel a pressured to make every Dom you find be a feasible play partner. If he isn't, you might not know when you can find play next. Personally, I believe a good Dom who truly wants to connect with a sub should have great patience and a willingness to slowly sculpt a boy. While it's certainly hot to have a "my way or the highway" type Dom, it's not always pragmatic when you're just beginning to discover yourself as a sub. If you find a Dom who won't exemplify patience, I highly suggest you do so in his stead and wait for someone who has more respect for you.