Just ask her 'What's in it for me?' by you helping her out with her coursework. You can ask this in a cheeky flirtatious manner but still mean the intent. So many mugs (me included back in the day) whilst at uni would help out the hot girls (its always the hot ones never the average and below) with their coursework not asking for anything in return but hoping you might be able to somehow manouvere your way in there. Don't fall victim to this. Chances are you'll end up being flavor of the week whilst helping her out with her studies then have your status relegated once the coursework is handed in.

Women aren't stupid they know if they can get something out of a guy and not give anything in return they will exploit that. It's human nature. Tell her she needs to return the favor somehow and leave that suggestion up to her, see where she goes with it.

You say you've been in the friend and comfort zone for along time now. This is risky, if you really value her as a friend you may have to face upto the reality you may lose her as a friend if you insist on taking things further. Pursuing sexual relationships with friends can get messy and complicated and having salacious urges towards a friend can ruin a perfectly good friendship. Personally I don't like to complicate things with women in my life I regard as good friends, I like to keep distinctions seperate.

My advice would be to seek women beyond your inner sanctum of friends but if you do insist on seeing where things go with this woman then I'd proceed with some caution. Like you say 'test the water' or perhaps ask her 'how she's going to repay the favor?' Just to see what kind of initial response you get then calibrate from there.