Not sure if certification is typical in Canada, but in the US I've never seen an instructor advertise as being certified, I've never asked about certification, and there's no governing body who issues certifications that I would care about even if someone was certified.
Actually, not entirely true, I could potentially care about USDF instructor certification, but there's literally 1 USDF certified instructor in my entire state. Given the number of well respected dressage trainers in the area who have not bothered to get that certification, I guess doesn't hold a lot of weight, either.
As Endiku pointed out, the biggest thing would be insurance. Be sure the facility you're thinking of teaching at carries insurance and what it would take for you to be covered (i.e. it may require you to be supervised for all lessons. That may be just a designated adult on the premises, or it may mean someone actually watching at all times)