Severity of the course
Understand your ability as a rider and look for a realistic challenge that will test your limits but one that is attainable if you put the right amount of work in. If you’re new to sportives then don’t get carried away and brainwashed by the word “epic” on the entry form. It may seem like a wonderful idea at the time but remember the further you are from home the longer it’ll take to get back should the circles you were effortlessly turning at the start suddenly turn to squares mid ride.
Look at all the elements that make up the ride including what options the organiser may offer to cater for all abilities. Check the amount of elevation for each and decide what is the most suitable based on your level of fitness and experience. As a rule of thumb a mountain stage of the Tour de France will have around 4,000 metres of climbing so use this as a benchmark when making a decision. Research the course, the climbs, where it’s possible to fill bottles or stop for food. Although riding the route in advance will undoubtedly help, in practical terms familiarising yourself with the event may come down to speaking to riders with past experience or seeking information online. If it’s the lure of the mountains and a European sportive that’s your goal then Cyclefilm’s series of recon videos may be of help.