How to check if the belt if fully loaded?

[Dunno]

I have belts going to my smelters, is there any way to create a checking device, that would inform me, if a belt is not full?

[Ramy83]

You could check with an arithmetic combinator if the amount of items on a belt is unequal to 8 and combine it with a lamp as indicator.

[The_Mell]

Just connect a speaker to the belt - or a lamp depending on how you want to be warned.

Here an example how i connected an inserter to take only stuff if belt is full on one side:
https://steamcommunity.com/sharedfiles/filedetails/?id=2287221769

[KaszkaManna]

Place inserters / fast inserters on both sides. See if they can place an item on the belt.
It's probably the easiest solution.

[knighttemplar1960]

Each section of belt holds 8 items. You can use a red or a green wire on a section of belt to have it read the contents of that section of belt. You can then hook that section of belt to an array of 8 lights and set the lights to come on in order if there are >1, >2, >3, >4 etc items on the belt section all the way up to >7 and then just watch the lights for a bit. If they all stay on all the time the belt is saturated. If not you can tell by how many lights come on how full or not the belt is.

You can do the same thing with a longer stretch of belt by having the wires read the entire stretch of belt and then divide the number of belt sections by 8 and have your lights light up in increments of 10%. Or if its close you can have it just check 90% and up giving each light an increment of 1% of the total.

You don't really have to be that precise though unless you are going for perfection. Eyeballing it is usually close enough.

Or you could just download one of the rate calculators.

https://mods.factorio.com/mod/MaxRateCalculator