As blinvir says, no visual. It’s on inspection only.
As for telling how many you’ll get - the furnace doesn’t work like other machines. There aren’t batches, there’s no bulk or mass crafting mechanic, and you can start and stop it at any time during the process.
If you stop it while it’s working, unlike other machines it doesn’t cancel the process and return the materials. It completes the current unit and then quits.
I go with how much do I want to make and then put the correct amount in. There is no visual notice that it is working, just that you check it to see if it is finished. It can give a countdown where you can see that the individual item is being “cooked” but it doesn’t tell you like a workbench will that it will take 4 minutes and when you check you see that you have 1 minute and 38 seconds till it is finished.
Going by memory, which isn’t the greatest, I think it take four meat and 2 lamella to make a meaty casserole. So if I want to make 10 of them, I just put in 40 meat and 20 lamella. One thing that I like is that when it finishes it just stops, it doesn’t keep burning the fuel it has in it.
Good tip. I remember when i started out trying to put in exact amounts of coal so i didnt waste any. No need to worry about it at all. Doesnt even keep burning the last peice its using.
Forgot to state, to see what is made, there is a box that shows the made item. Put in copper ore to be smelted and you get one bar for each ore. So, if 30 ore, 30 bars will be smelted. You can see how many have been made when you look at the bars.
Those are things that I put in to cook, whether it is ores, food, or what ever and go mining, gathering of items or even end the play time and leave the game. It will continue the process.