There was a discussion in comments on my previous average last frost post on how to determine an accurate last frost. A great point was made by Daphne where every garden can have its own microclimate so the date you find online or in a book could be many weeks off for your microclimate in neighborhood.
To be able to predict your last frost date accurately it takes some history, if you have lived in your current residence and happened been capturing your temperature data accurately by memory or paper you may the information to predict your last frost date.
For people that are relatively new to their residence, like myself, I felt there I had very little information to determine an accurate prediction of my last frost date. This was until I remembered Weather Underground which logs results of schools or private individual that register their weather stations.
After a quick search I was able to find someone who had a weather station in my neighborhood. After doing a custom query of the full range of the of the weather station, which in my case was a little over 4 years, I had all the information I needed. By carefully adding a horizontal line at freezing using a graphics program (Microsoft Paint) I could easily see where the low temperature dipped below the freezing mark.
From a quick glance at the data it appears that the first week of April is a pretty safe date for past few years. But if you look the the irregular low temperatures we have had this month this might be harder than I thought.
Now of course your neighbor may a different microclimate than you, but should get you in a better ballpark than the generic number you find fro your city. You could always spend $500-$1000 on your own weather station to get some better accuracy, but for me seems close enough to me. Though the weather station would be a really cool gadget to have.