David Rothman does a great job of highlighting some interesting drug interaction checkers (click here).  While they can never be 100% relied upon they can prove helpful. 

I tend to use a Medscape resource (click here) but you need to be registered with Medscape to use it.  But when checking for drug interactions I tend to check these resources:

• The SPCs
• RxList
• Drugs.com

As mentioned above I don't think any method is foolproof.  Caution seems sensible as I doubt the capturing of interactions (as with adverse events) is hugely accurate.