Indoor Pest Control - How to identify/deal with unseen biting bugs?

We have a detached two room converted garage that is our guest building. When my dad and his fiance visited last fall, she complained that she was getting little bites on her arms and legs at night. My dad was unaffected. We figured it was the blanket she was using (as she had it wrapped around her as she read in a chair at night). Later, when we described this to my in-laws, they mentioned that they get the same thing every time they visit, but had just never told us!

We have never been able to see anything, but don't really know what to look for. Could this be bed bugs? My wife did some reading, and thinks that it might be rat mites (the structure is post and pier, and the underside is easily accessible to critters).

How should we go about identifying what is going on? We have not had a visitor since October, and the building has been unheated through an unusally cold (for Seattle) winter where we were below freezing for a week or so. I assume that with bed bugs, they would still be out there. If it is rat mites or something, there might be no sign of them, but they might come back if there is someone sleeping there.

Thanks,
~john

Since it is a stand alone building Rent an Ozone generator (http://www.ozoneapplications.com/rental_programs.htm?gclid=CJOj7_WIvZgCFQIyxwodTjekZA) and run it in the building for a few days.
It'll kill about everything. Please use caution and abide by it's safe and use rules.

You can strip the bed and look for signs of bed bugs and their fecal matter. I don't think you will find any symptoms of bed bugs. Clusters of bugs and dirty fecal mater.

You can run the recommended ozone machine as recommended in last post, but stay out of room and air before occupying. Do not be in room when ozone machine is running. This will only address current 'bug' problems.

If you live in a humid and damp climate or you do not monitor humidity and it ranges high, you may have gnats. Address moisture issues around structure. Monitor humidity with hygrometer (sold where thermometers are sold). Humidity should be maintained 35-55% year round. If too high, dehumidify with dehumidifier. If too low humidify. Humidity tends to vary from room to room with kitchen and bathrooms with highest humidity.

Gnats may be breeding in drains, overly wet flower pots, refrigerator or air conditioner drain pans. Clean and disinfect.

I did a little research on Ozone Generators, and it seems as though they don't have a 100% kill rate. Also, if I can't identify what's there, I can't know if I got them all until someone actually sleeps in the room.

Is this really my best bet? Should I contact a Pest Control service? Will they be able to do anything other than just fumagate?

I am guessing that things like steam cleaning the carpet are kind of pointless, right?

Thanks,
~john