Is there any difference between an AR15 bolt and a m16 bolt.  If so, how do you tell?

Thanks for your help

Bolts, no.  Carriers,yes.

Dimensionally there's no difference between an M16 and AR-15 bolt.  The M16 bolt will ALWAYS have been magnetic particle inspected, while a commercial AR-15 bolt may not have been.

A quality, name-brand (Colt, Bushmaster, Armalite) AR-15 bolt will be identical to a military M16 bolt.

Only Colt carriers and bolts are truely mil spec and gar-un-teed to me MP tested.  All "AR" and "M16" bolts will interchange and the Colt's all come from the same batch (mil and commercial).  The carriers are very different.  "M16" carriers are for auto/burst function.  If the the tail end has equal cut outs on top and bottom it is a 16 carrier and you don't need it in a semi system.  If the bottom cut out is bigger or there is no bottom then it's a semi carrier.  Again only a Colt "M16" carrier mil spec.  The Colt commercial carriers are going to be equal in quality regardless of style, they just were not made for the govt. The only other mil spec stuff is FN which doesn't sell to the public.  Other brands are not mil spec...period.  

Advice...a few extra $$$ is worth it to stick to Colt.  You know what your getting and it's is the best.