So how the player is supposed to mix potions and craft gadgets: by pure guess or there's some ingame hints?

This is a good site to find lots of that kind of info. http://www.zimlab.com/wizardry/flamestryke/wizardry8/flamestrykes_w8.html

Thanks, but the question is about ingame knowledge.

There are no ingame hints for either of these BloodCat which maybe due to the fact it's the last in a long line of Wizardry games. As a newcomer to the Wizardry series over a decade ago, I jumped in with Wiz 8 and found there was a lot I had to work out for myself. For me that was part of the fun of the game.

However, here's some info which I hope will assist;

In your inventory, sort everything into order and all the gadget items will be together. Select your Gadgeteer and drag items onto one another to see if they can be merged to create a new item. Don't be surprised if you find only a few fit together as you're intended to search and engineer gadgets throughout the game. If you find you've created a gadget your Gadgeteer cannot use, there will be information to tell you what level of engineering skill etc., your Gadgeteer needs to have. A Gadgeteer is a useful character and I found merging and creating items fun.

It is a long time since I've played using an Alchemist, but if memory serves me right potions are created whilst you rest. I'm sorry I can't tell your more, but hopefully someone else will be able to assist you.

The web link given by ussnorway is a useful one for information.

I hope you enjoy Wiz 8 and have fun with it.

Well, that's an answer! :) Thanks. I've played Wiz8 about ten years ago, but due to RL-issues never managed to get past Arnika. But I still remember how many times my party was slaughtered on the road from Monastery. :)
So, it's trial and error then -- fine by me. Do I get a message if the items I'm merging can be merged, but my skill is insufficient? Flamestryke's guide is awesome, but I'll try to figure most things myself, cause it's more fun.

You will get the message "xxx cant merge those items" (or similar;) if the highlighted characters skill is insufficient,
in oppostion to "those items cant be merged" if it is impossible Yesss, that isss the ssspirit!