This group I tend to associate more with Upper Palaezoic rocks, based on UK fieldwork. Rugose corals have strong septae, as you can see in this typical solitary form:
A solitary rugose coral (Zaphrentites spinulosum), Lower Carboniferous and found in Tennessee. ~3.5 cm long.
If you find something like the above in a Palaeozoic rock, it's a rugose coral (tabulate ones are colonial only, remember). But rugose corals can also be colonial! Here is an example:
A colonial rugose coral. This one is Acrocyathus floriformis, and it's Lower Carboniferous in age (Mississippian St. Louis Limestone of Monroe County, Illinois).
In particular, note that you can still see the septae of each corallite quite clearly – then compare to those tabulate corals below.
Tabulates were amongst the earliest corals, and are our Palaezoic, colonial only group. In contrast with the above colonial (rugose) coral, check out the one below. Note that it doesn't really have septa when looking down the calice. This is typical of the tabulate corals.
Tabulate coral Favosites tuberosus. Devonian in age, and sourced from the Onondaga Limestone, Erie County, New York. Longest dimension of specimen~12 cm.
On the sides of the rock you can make out the tabulae within the coral where the corralites are split. Sometimes there are chain like, branching and tubular forms of tabulate coral. Here's an example of the latter:
Tabulate coral Thamnopora limitaris from the same site as the last fossil. Maximum dimension ~17.5cm.
Let's finish with a cheeky scleractinian. This time a solitary one. See if you can get a feel for the less chunky skeleton than you see in solitary rugose corals - or spot the difference in the septal arrangement.
Extant solitary coral Flabellum moseleyi. This lived at ~250 m depth off the southwest coast of Florida. Specimen ~5.5cm long.