We know because of the verb's ending: "yeriz" is the "biz" form of "yemek." (Ben yerim, sen yersin, o yer, biz yeriz, onlar yer(ler).)
Does the k at the end always bend to ğ instead of g, when e.g. in accusative?
How do we pronounce the ğ?
In general, not at all.
If anything, it's a little glide between vowel sounds -- so between front vowels (e i ö ü) it's a very light "y" sound and between back vowels (a ı o u) it's a very light "w" sound.
ekmeği sounds a bit like ekmei or ekmeyi; doğu a bit like dou or dowu; etc.