I used to just find leftovers in the fridge when I was living with my mate for a little while, if it tasted like it should cold I warmed it up, if not I binned it.
I remember eating old chicken chow mein once, then I remembered and I was fairly sure that it was mine from two weekends ago, no mould or anything, tasted okay too. No food poisoning.
Generally, if you warm it through properly, you shouldn't have any significant problems.