98 Venture...heating problems

Hi everyone....just got my van back from the shop. Took it in last Oct., problem was intake gasket. To make a long story short they ended up changing the entire motor. it now has a motor out of a '00 Alero.
My problem now is that i'm not getting any heat, front or rear. Temp gauge doesn't move much, if at all. If I do alot of stop and go driving the gauge will move a bit, but never into the 'normal' range. I've been told it could be the thermostat or the water pump. Is there a way to test these before I take it back to the shop or undertake the job myself?
Any thoughts would be great.

Thanks

my guess is either there is no thermastat in it or the t-stat that is in it is stuck open. replace the t-stat with a new one made in the usa. do not put a cheap t-stat made in mexico in as you will just end up changing it back out.