Identifying orchids can be quite tricky! Some species, although different, can be very alike. Other species can be quite variable within their own kind. Some species will hybridise freely with similar species growing nearby. All of these are an issue when faced with either heath spotted orchid (Dactylorhiza maculata) or common spotted orchid! These two species are not the only ones to have spots on the leaves either ... and they hybridise with marsh orchids.
In general, however, heath spotted orchids are usually found on acid soils, the type of soil that forms our heathland whereas common spotted orchid is more normally associated with chalk and limestone grasslands and meadows. So location is a key here. However, the only sure way is to look at the lower lip of an individual flower in the spike. The heath spotted has a broad lip with a rather inconspicuous tooth whereas common spotted has three teeth on the the lip. The southern marsh orchid lacks a pronounced tooth on the lip, generally has no spots on the leaves and is usually found in damper environments. Hybrids, however, can really confuse the issue!
Heath spotted are usually paler in colour than the common spotted but common spotted can even be white sometimes, southern marsh are usually a much deeper purple than the other two.
So, spotting the difference can be quite difficult, especially for the casual observer like me.