Are there anyway key ways to find out how long a property has been on the market when looking though loopnet.
What other sites are a good when looking for comerical real estate?
Short of asking the realtor, there is no way (commercial properties are rarely listed on the MLS).
Here are some other sites you might want to check out:
Loopnet is the biggest of all the commercial real estate websites. Perhaps you could see when a listing was last cached in google, to give you an idea of how old an ad listing is there.
On loopnet it says when a property was entered (not the date of last verification). It is on the bottom right hand corner under the debt and equity info box. Now I guess is won't tell you if it has been listed on there before and withdrawn, but it should help a little.