A few pointers... stay away from:
* Unusual signed items - baseballs, flags, etc. 99.9% of them are fake.
* Items of opportunity - these are items that someone finds at a garage sale for 25 cents and is a "cheap investment" as forgery fodder. Examples include, postal covers, signed buttons, signed pamphlets, signed postcards, signed books that are not the books he would sign through his office, etc.
* Items with "too good to be true" inscriptions such as "Tear Down the Wall", "I Forgot to Duck", etc. Almost all of these are fake, and beware of signed typed quotes. There is a guy on ebay that has been selling these for years... I think they are all fakes.
* Unpersonalized signed photos in the perfect pose (flag in background, etc.) These are almost always fake.
* Books that are signed (not on a bookplate). Reagan never did any booksignings and books sent to his office were affixed with a bookplate. Authentically signed books are very rare.
If you have any items you'd like me to take a look at, please leave a link in a comment and I'll look into it.