This is ultimatly gonna be one of those things that you will have to trust your own judgement on. Some things that I would consider are the strength/integrety of the wood. Is it still nice and solid/hard or has it turned soft and eaten with bugs or decay? If split, is the inside still clean and fresh looking or are there signs of mold or fungus? Does it smell bad before or during burning? Colors of grey don't excite me, white, fungus or black slime might.
I don't think that there is a majic number on the time that your smoking wood will last before it "goes bad". I would also have to say that two years well covered, off of the ground and fair to good ventilation is just becoming "well seasoned" wood depending on the type of wood and even the size ov the peices. I would guess that air dried only would take your would down to 20-30% MC, good for smoking, bad for building interior furniture. Dry enough to burn, but wet enough to smoke and not "Flash catch" quite like tinder.
All of that being said, I am no expert. Some woods may loose their smoke flavor after too much time, or worse, leave a bad flavor. I have limited knowledge of this because I use mainly Hickory and Oak. There may be some others with better expeirence on this or those that may have heard some old folklore about it. (I am not knocking folklore, I am going on the assumption that it is rooted in years of experience and oraly passed along)
Pretty much, if it smells good I would guess it to taste good. If it has a bad smell, I would bet that it leaves a bad taste.