A good way is to use chronometer or myfitnesspal and keep a food diary for a typical week. This will help you track how much protein you are getting each day. You don’t have to do it for a long period of time but during a typical week will help you to establish your training needs. I do feel there has been a huge push by the fitness industry to make everyone think that they need to supplement with shakes. Bodybuilders or those trying to put on muscle can help but for most athletic people your daily protein requirement can easily be hit by a good wholefoods diet. I’m vegan and I get plenty of protein thorough my normal diet but will occasionally use a shake if I’ve done a very long ride or hard session, strength training or know I’ve had a day where I haven’t had a lot of protein.
One point to note, the post workout window has been shown to actually not be true. There is no significant benefit to taking a protein supplement post workout than consuming protein throughout the day. Carbs to refuel yes, after your workout but protein its ok throughout the day. However, in your case with fasting, I would say that post workout it is essential that you have some protein. It may also be a good idea to consume some protein prior to sleep.
So track your nutrients, this will tell you if you need to up your protein intake. Try and do it through whole foods rather than supplements as you can get additional benefits such as fiber and micro nutrients if it’s through sources like pulses and legumes. There are a number of vegan protein shakes now that can be easier on your stomach. Whey protein can have some nasty side effects in some people and natural sources are so much cheaper for more benefit. The companies selling the protein talk about maximising your post workout window, but this is just marketing talk to get people to hand over their hard earned when they can get it in a much cheaper more nutritious form.
However, protein shakes do have their uses and I’m not against them at all. It can be easy just to swig one quickly if you haven’t got time time to have a proper meal. But people swig them like they are some kind of miracle cure when in fact you can get by quite easily without having to purchase them.