how can you be the girlfriend type not just fwb

The only good advice here is from the most recent post! Just skip all the rest, I don't know what those people are thinking...
I agree with adam_k : tell the guy you like that you are interested in something serious. If he's not interested, then you don't need that guy!
I think there's a lot of bs on the internet and out in the world, ideas about how women should behave that have little to do with what women want or finding a good person to spend time with. The only way you're gonna know that someone is interested in you for a committed relationship is if you ask them. You could ask on the first date: if they're just looking for somebody dtf, then you don't have to waste your time.

A guy who is a interested in something serious cannot take his eyes off a girl, so there is no need to ASK: it is self-evident by the multitude of non-verbal signals an interested guy is sending.

If you have to ask, then you might as well not ask and simply not consider the guy at all - why waste time on guys who are ambivalent?

Plus, asking this kind of a question would portray the OP as desperate. I think that the general rule is not to be desperate, but if you cannot help being desperate, at least not act desperate and sort of fake it until you make it.