The best way to determine if your red oaks have oak wilt, is to have a certified arborist or forester examine the trees or you could submit a tree sample to the MSU diagnostic lab.  The web site for the MSU lab is www.pestid.msu.edu where you will find instructions on how to prepare and send a sample.
Below are some government web sites with info on the disease cycle and pictures of diseased red oaks.  Hope that helps!

 http://na.fs.fed.us/pubs/howtos/ht_oakwilt/identify_prevent_and_control_oak_wilt_print.pdf

https://www.michigan.gov/documents/dnr/IC4166-C_256344_7.pdf