You need to map the properties in the immediate market area you are looking to purchase in and do a market survey. Contact each property by phone, fax or e-mail and ask for their current occupancy percentage, prices,etc. Take the ones that are in the same price range and look at their current occupancy rate.
If occupancies in your market area are below 92%, there is a surplus. Also take into consideration the unemployment rate in the area compared to the national average and if it is a college town. If you check occupancies in a college town from May-August, they could be typically lower than in the other months.
I would buy in a market where occupancy percentage in the months of May - September are above 92%.