How do you get the activations number?

Now that I have decided to get a used 40D or 50D, how can you tell how many clicks or activations it has?

Plenty of threads on this already.
Hit the search button or go right down the bottom of this page and have a look in one of the threads listed there

Edit....

Just use the camera and don't worry about the actuations.
A shutter will give up when it wants too.
In my experience, I've replaced 3 shutters over the years - 2 in a 10D and only two weeks ago the one in my 1DMk II, they have let go well over their rating but have heard stories of them going at less than 10k.
Don't have first hand knowledge of this though.
Again, there's plenty of stories on here about that happening.

The number of actuations basically will tell you very little about the camera.

Shutter life is highly variable and can range from 5,000 to 150,000 (you are incredibly unlucky if you get 5,000 and it goes).

I would go on the signs of general wear and tear (scratches,nicks, wear etc) rather than shutter count as these will give an indication as to whether the camera has been well treated or not.

Saying that a Pro will be generally harder on his gear (the camera is just a tool to him) than an amateur (to which it is his pride and joy) so ymmv.

Ac-tu-a-tions

like buying a used boat- lots of hours and not a scratch, probably a cruiser that had a special place in a heated garage. low hours and beat smooth on every corner, a thrasher that went from the trailer to the fishing hole, spent the entire day there, and beelined back at dark.
It makes it hard to decide-whats more important?

you rarely get more than two of the following:
1-low actuations
2-great condition
3-reasonably priced for a camera that isn't under warranty anymore

Always remember, everything is for sale for a reason

thanks