What do producers usually mix ground coffee with to make it cheaper? I will not go into details, or reasons (political, economic, logistic or otherwise), but in my country, coffee has been scarce and expensive in the last couple of years.

I've been suspecting that some coffee producers have been cutting it (mixing it) with cheaper things. The unmistakable aroma of coffee is now mixed with some bitter tones sometimes, fishy tones some other times. The flavor is not quite coffee also.  It's been so long since I've smelled the unmistakable aroma of real,  honest to God coffee. I'm almost 100% convinced that they are mixing it with roasted black beans or something.

My questions is, what other grains, nuts or whatever do coffee produces  usually mix ground coffee with in order to make it cheaper? 

How can I check whether or not the ground coffee has been mixed up?
I will not go into any political discussion, theory or similar, but I can give you a few historical facts.

There have always been times in the last few centuries when coffee was a rare commodity. And it has always been expensive to some degree, making it "something for special occasions" in the poorer parts of European societies.

People used and still use various "substitutes" from grains (barley, spelt, rice), roots (chiccory, dandelion), fruit (acorns, dates), typically roasted, ground and prepared like coffee beans, resulting in a somewhat bitter, dark drink more or less resembling genuine coffee. (For a more extensive list, see the link above.) 

Instead of replacing coffee, they can also be mixed with coffee, "stretching" the yield of the coffee beans.

If you want to check your coffee for "fillers", buy whole beans - none of the coffee substitutes look even remotely like coffee beans.