I have a set of 99's that I think are PI but want to make sure, how do I tell?

I have been looking all over and can't find any info on the visual difference between them.

Thanks.

EDIT: They are off of a 4.6

 

Assuming the heads are off, look at the pic in the upper right corner of the head for "PI". The red arrow was for the oil plug.

 

Thank you, rep sent.

So if it says nothing in that corner than it is the older NPI?

 

non-pi heads have a ridge in the combustion chamber.

 

Ok, now I will know what I am looking at when I pull them. Thanks.

 

These are off a 5.4, but I think they are the same heads. There's a couple places where it says PI, the only one you might see with the engine in the truck is near the exhaust ports:



the intake ports on a non-pi head are pretty round:



on a power improved head they're more rectangular:

 

I found today that some PI heads are not marked.
The best way to tell is by looking at the shape of the intake port.