How to find a given text's complexity?

Yes and no. There are a number of well-known algorithms for determining a text's complexity. There are several generations of these and most of them are tied to educational attainment such as years of education, education level studied or age.

They are very much replicable across populations but they do not do a very good job for determining suitability to an individual and/or very short texts.

Most of the algorithms take proxies for semantic and syntactic complexities in the length of words and sentences. They do not look at phonic complexity - which may have to do with things like consonant clusters, etc. but would require more work. The Wikipedia entry on Readability does a good job of summarizing them.

The first generation of these algorithms peaked in the 1960s and 70s and they have been built into many mainstream tools such as Microsoft Word and little free tools can be found online. Since the late 1990s, there have been some efforts in second generation products - see a review by Renlearn (which is a commercial actor in this field so keep that in mind).

The lesson is that you cannot really just make up an algorithm without testing it against readers - and that adding factors to the measure does not necessarily improve reliability.

Things get even more difficult when you try to personalize the measure. You need to know a lot about the user - with beginner or struggling readers, it's their phonic skills which is something we've tried to achieve in our work on the Phonics Engine but then you need a way to really understand the text at the phonic level.

You also need to take into account the reader's interests because if they love dinosaurs a word like Pterodactyl may be actually easy for them. Ultimately, if you want to help individuals, you don't need a tool to help you categorize the texts but to help them access the texts or to help creators create more accessible texts. The tools for this are still not very good and looking at a single dimensional score for texts is more of a hindrance than a help.