Recently a structural pest inspection showed evidence of "powder post beetle infestation in the substructural framework." The house is 40 years old. I read that this type of pest usually is found in the wood that the house is built with and that it well could have been an inactive infestation. How do you tell? Could it be that this pest has been in my home that long? Is there an alternative to fumigation?

It can sometimes be difficult to tell an active infestation from an inactive one.
The color and texture of the frass (the sawdust-like substance resuling from pest invasion) can be a clue. Does it look fresh?
Because they work slowly, you have a little time to do another test. In a 2 or 3 foot section of the wood showing the most damage, plug every hole with wood putty.
If more holes appear (it may take 3 to  months) the infestation is active. As for alternatives,  Timbor  can be sprayed on unfinished wood and it works well, but is only useful on exposed unpainted wood.