To figure out if your site was hit by Penguin or if it was Panda or another penalty you need to check your traffic on the 24th/25th/26th. Penguin was released on the 24th April but updates sometimes take a day or so to roll out across data centres. Make sure you are only looking at your organic, ideally filter down to google traffic if you get significant amount of organic traffic from other search engines.
A few things to keep in mind:
- There was a Panda3.5 update that dropped on the 19th April so if you had a traffic drop around the 19th/20th it was probably due to Panda.
- Then there was another panda update on the 27th so be sure about your dates.
- Always watch out for seasonal changes in your market/niche.
- Be aware of other general google updates, during April they listed 52 "Search Quality Highlights".