How to detect specific item in gui slot.

I am not a modder nor a very good coder but I took python and java in high school. I am making a weather mod that detects if a water bucket is in slot 0 and runs toggle downfall command. I was able to do that just fine but i can find a specific block detector, im looking for a block that is maybe like if(Item in slot 0 is "water bucket"). But I cant find one.