There are several apps that will check your available RAM and other stats for the iPad. I use Systeminfo for iPad (the free version). I'm not sure what the paid version offers.
If you start the app then access the multitask bar you can watch the RAM increase as you close apps. If you have enough apps in the bar, eventually you stop seeing RAM increase. That is because those apps were already automatically removed in order to free up RAM when it started to get low. Apple probably chose to leave the purged apps in the multitask bar so that you can still open them easily. After all, most of them also save their run state, which means they will open where they left off, much like the ones frozen in RAM, only slower.
On the iPad 1, never found more than 5 to 7 apps in RAM. On the iPad 2 I've seen as many as 18. The extra 512MB makes a big difference.