In general, accurate tick identification of adult and nymphal ticks requires examination of the specimen under a stereomicroscope (at a minimum). Numerous identification keys have been developed to help with this process (see below). Larvae need to be mounted on a slide prior to microscopic examination.
That being said, it is still possible to get a good idea of the species you are dealing with for some unfed adult specimens by careful examination with the naked eye. We've provided some labelled photos below (not to scale) to point out some key features.
This schematic drawing highlights some of the key anatomical features that can be helpful to differentiate tick species.
Blacklegged tick / deer tick (Ixodes scapularis)
[In western Canada, the species is the Western Blacklegged tick (Ixodes pacificus) which looks the same to the naked eye, but can be differentiated microscopically]
Groundhog tick (Ixodes cookei)
American dog tick (Dermacentor variabilis)
[In areas of western Canada, the predominate species is the Rocky Mountain wood tick (Dermacentor andersoni) which looks the same to the naked eye, but can be differentiated microscopically]
Lone star tick (Amblyomma americanum)
Brown dog tick (Rhipicephalus sanguineus)
Tick Identification Keys
If you would like more detailed information on tick identification, including identification keys for use with a stereomicroscope, A Handbook to the Ticks of Canada is a great resource.