Your English seems very strong, so try to picture the word as itself. Can you count it without a counting word? Does it accept a/an? It's probably countable. Uncountable nouns require a countable word (a piece/bit/chunk/etc. of information.) in order to talk about plurals (excepting in conversational cases where you can have "waters" or "beers" etc). Your examples: Recommendation - if we take this as, for example, a letter of recommendation, you can receive multiple recommendations. You can physically count them and it is not strange to try to do so. I need four recommendations to apply to the university. It is strange to try to add a counter and it accepts the article A just fine. Suggestion - You're right, it's countable. You can have many suggestions and you can have three or six or 100. It is not a concept that is difficult or confusing to deal with in quantity. Picture it like rice. Rice is uncountable because there are so many small pieces that make up the rice. To talk about one rice is strange (What do you mean one rice? One bowl? One cup? How much rice is one rice?), so we need a counter to attach to it - a grain of rice, six grains of rice, a bag of rice, etc. The rest of your examples are all countable as well. To my knowledge, there is no specific rule for defining if a word is countable or uncountable based on spelling - it is based on the word itself. But then, this is English we're talking about. There are excepTIONS all over the place.

the only way is memorizing or checking in the dictionary