Hey there, chrysanthemum. Happy to help you with this. Welcome to the boards.
Something to know about this first and foremost is that you actually won't even likely *need* to know the difference. You will know for yourself if you feel sexually excited or not, regardless of what your discharge looks like, or if there even is any. It's not like we need to know to be able to tell if we feel turned on or not, because that's not only about more than our bodies, but sometimes we will be and our bodies won't do things like produce fluids.
That aside, vaginal discharges that are part of the fertility and menstrual cycle, part of the way the vagina keeps itself in balance or both will generally tend to be thicker than fluids from sexual arousal (sometimes called vaginal "sweating" because it is often a sweat-like consistency). While fluids for all those other times vary -- they can be anything from thinner and clear to thick and white and more paste-like -- but will tend to be thicker and less clear, if and when they're super-present at all. Sometimes they're not.
Around ovulation, that discharge gets thinner and clearer, but tends to have the consistency of egg white -- it's not usually thin like what we can see with arousal.
Does that help?