I have an old Marlin 44 magnum and to tell you the truth. I can't remember if it was micro-groove or not. I sold it because I couldn't get Keith style cast bullets to feed due to the length of the loaded cartridge. But I'd guess that ghostman is right.
I have a new Marlin 44 Mag, that I really like. It was manufactured in the last couple of years. It's accurate with jacketed bullets and I've learned how to modify the feed so that it will load the longer bullets. It has Ballard rifling.
Some of you missed this part evidently:
Other than inspecting the rifling is there any way to tell? It seems that I once had a Marlin with Micro-Groove rifling and it said so on the barrel but the memory fades with age.
LDHunter