How to tell the difference between a 1098cc and...

[HWood]

....something smaller just by looking at it once it's removed from a car?

Also, what should I be looking for when buying an engine that's already been removed from a car to make sure it's all okay?

[mike.perry]

1098 has 1100 on a plate behind the exhaust manifold and has a thick steel back plate. The clutch is a lot larger.
948 has 950 cast on the side of the block, has a thinner back plate turned up around the edges and a smaller clutch.

[Matt]

803 has a spin on oil filter screwing in to the side of the block

1275 has thermostat mounted at a different angle & do no have removeable tappet chest covers

[dunketh]

Also, what should I be looking for when buying an engine that's already been removed from a car to make sure it's all okay?

A good price!
Seriously, not sure you can tell big stuff like main bearings etc.. without actually running it up.
Anything else is likely to be easily fixable anyway.

[bmcecosse]

You either take the seller's word for it - or walk away.... Obviously if it is thick with oil/muck you are going to be less impressed than if it is nice and clean (but not TOO clean...) with no obvious oil leaks. You can dip the sump to get a feel for the oil in it....and pull a spark plug to see how it has been running.