So how did I make my choice (a company called Sportavia)?
First and most important, a search (on Google). Sportavia wasn't top, but was mid-way down the first page - high enough to guarantee inclusion in my 'possibles' list.
Second: review the 'possibles'.
Comparatively, Sportavia's site was the most professional-looking, with relevant and easily-found information, which implied an efficient and well-run organisation. I felt I could risk committing my hard-earned cash before I'd ever set foot there.
Third: gut feeling.
Sportavia's site oozes a laid-back, friendly professionalism, e.g. a diary that's updated daily - this really lets you get the feel of the place and of a typical day.
Fourth: how do I book?
A simple and well laid-out form means I don't have to jump through any hoops to give them my money.
So, four steps to success. I had a great time there and can thoroughly recommend it. Simple really, and I wish more sites were as honest, enthusiastic and functional as this!