Is it possible to explore somehow what kind of planet is in the slot or should I waist the colony ship anyways and then find out that planet I found junk (too small, or too hot, etc.)?

Exploration technology is for sending any kind of ship to the 16th slot to search for resources and more ships. Its a chance, since the ships you send can be destroyed. No, you can not see a planets fields or tempurature before you colonize it. So yeah, you will have to what you called "Waste a colony ship" to find out what the planet's atributes will be.

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Thank you very much for the answer!