I have 3 Pre-Ban AR's 1 Bushmaster and 2 Colts.  How do I tell if they have a sear block and where should I look?  

Take the upper off, look at the millied out section just rear of your trigger, if it is the same inside diamater as the part your trigger/hammer/ect is, then you have NO sear block, if it tapers to a smaller inside diamater as it goes rear you HAVE a sear block.  I'm sure someone who knows more than me about these computer thingies can post a pic for you.

push the rear takedown pin all the way to the right and breakover the upper receiver so that you can see down into the lower receiver. If you have a sear block it will be directly above the safety lever. You cant miss it if its there, and bushmaster doesnt put sears blocks in thier lowers they just make the inside area where the sear would go about 1/8 inch too narrow for a usgi auto sear.