This is going to be tough to be sure - much of this stuff is produced in factories with unknown standards for the use or non-use of toxic materials. Always better to be safe than sorry.
That said, I'm by no means an expert on ceramics and glazing but I do know that prior to firing, glazes are most definitely not safe. There exist specific risks related to dry glaze mixes. Certain chemical elements are used during glazing to effect particular colors and as part of the process. Certainly of concern to those using glazed dishes is the presence of lead in the glaze. Cadmium is also called out in the literature on glazes. And, of course, many other substances could be harmful in sufficient enough quantities.
After firing, glazes tend to be much more stable, particularly those not subjected to the microwave or dishwasher. It would be my (uneducated, non-expert) opinion that if one were worried about the off-gassing of toxin from a glazed ceramic piece, leaving it outside for some period of time would be prudent. Off gassing, if it is going to occur, will likely happen under those conditions (where temperature fluctuations are going to be wider) if it is going to happen at all.
I believe there are kits that one can use to test for the presence of lead at the hardware store too.
I suspect the toxicity risk is likely low but how do you really know short of having a material safety data sheet that lays out the facts about the product?