What Cassi said. You have to know your market well.
My most recent purchase is a duplex in great schools and a hot area. I saw it within the first 20 minutes of going on market, it was being sold as is. I worked the numbers in about 7 minutes just to confirm my rule of thumb, called the listing agent to confirm if they were currently leased at what I thought they should bring for that market, sent a full price offer without waiving my right to inspection. Found out that evening it had multiple offers and it went to a "Highest and Best". I upped my offer by 3k, wrote a personal letter with a family photo, (did this because when speaking to the listing agent I found out the tenants cried when they learned the current owners were selling, so I knew emotion might be in play and wanted to let them know we are a young local family that care about our tenants). Low and behold, we got the deal and we were NOT the highest offer. Inspection turned out great, I got a nice little duplex with long term tenants in it who do take pride in ownership. All of this from knowing the market well enough to realize we had to move quickly.