Are adjunct ingredients always GMO by default? Does the presence of adjunct ingredients like rice or corn imply that these ingredients are always of the genetically modified variety?

I mean, if you are going to add adjuncts to a beer, the main reason is usually to reduce production costs right? In that case, GMO rice and corn adjuncts are the cheapest, so it seems to me like any beers using adjuncts will be using GMO adjuncts.
The chances of corn being grown for industrial purposes, like making dextrose have a very high probability of being GMO. If you are buying specialty grains (like for steeping), the probability of those ingredients being genetically modified are much lower. 

That said, finding out whether or not your adjunct is made from GMO corn (or rice) is very difficult. You may be able to find companies that market things as GMO-free, in that case, they are probably GMO free. Otherwise, they may or may not contain GMOs. 

If you are concerned about the safety of genetic modification, you should know that GMOs are very safe and are tested for safety much more than the wild crops their genes are borrowed from.