1. Is your product certified?
Certification is required, but some vendors try to slip through without doing the right thing. The profibus.com Product Finder shows you which products are certified, but not every vendor includes that information there. If the certification number is not there, ask to see a copy of the certificate. A certified product will be interoperable with other devices on the network, will respond appropriately to a power outage, will not generate an inappropriate network load, and will do much more correctly. Insist on a certified PROFINET product!
2. Where can I get the GSD file?
The GSD file is like a printer driver; it allows the configuration software to know what the device hardware is and what the device is 
capable of. It might be available in the profibus.com Product Finder, but there are better places to check. It might come with the product on a disk. By disk, I mean CD; if it is on a 3.5-inch floppy disk, examine the space time continuum for a rift. The GSD file might come on the product itself; the product may provide an internal web page with the GSD file included. Most likely though, it’s available for download from the vendor’s website. Ask your vendor.
3. I need this certain feature; does this product have it?
A number of PROFINET features are optional. If one or more of those optional features are important to you, ask if they are available. For example, if you are including IO modules on a robot arm, you may need Fast Start Up. Fast Start Up allows for quick connection to the network on a changeover. Other options include PROFIenergy and PROFIsafe.
In my experience, the vendors are knowledgeable; many have PROFINET Certified Network Engineers on staff. So go ahead and ask them about their PROFINET products.
–Carl Henning