There are many ways to assess your students quickly without the expense of buying a set of student response devices 'clickers'.

Socrative: "Socrative lets teachers engage and assess their students with educational activities on tablets, laptops and smartphones. Through the use of real time questioning, instant result aggregation and visualization, teachers can gauge the whole class' current level of understanding. Socrative saves teachers time so the class can further collaborate, discuss, extend and grow as a community of learners."

Poll Everywhere: "Poll Everywhere lets you engage your class or audience anywhere in real time." There are free plans available, as well as paid accounts if you want more features.

The Answer Pad: "It's all about student engagement and timely feedback. Interactive gives you the spontaneity to engage with your students at the time of instruction, giving them an opportunity to show what they know."

There are other choices I haven't personally tried out. Here is a list in case you find one that better suits your teaching style.

Lecture Tools
Learning Catalytics
Turning Technolog