there are many tools on the internet that will try and predict what your pr will be but they are far from accurate
I think that there is no way to find out about Future PR. No tool can give any accurate answer. These days, Google is updating PR very often, so you just need to wait for few weeks for another PR update.
There is no guarantee those PR prediction tools will give you any accurate results, but you can use them just for fun However, here is a link: pagerankprediction.com
Google always change their algorithm on SERP. It's almost impossible to predict our sites' future PR.
If your site is improving its postion in serps then you might expect and improvement in PR as the PR update is just and update of the google toolbar that reflects a gradual change in PR!