It is possible to detect Erk1/2 by immunofluorescence. There are specific antibodies available. Be sure that the antibody is highly specific for your protein as you cannot distinguish between correct or false signals in immunofluorescence. Dependent on your question, it is advisable to look for phosphorylated Erk1/2 in addition. There are also specific antibodies for the phosphorylated protein.
Kasturee: To add my two cents to Peter's excellent answers: while antibodies against total Erk1 or Erk2 can be done at any time, the detection of P-Erk1 or P-Erk2 is also time-dependent. It is therefore also advisable to know the timing of phosphorylation after stimulation and/or prior to fixing in your cell system. Good luck.
Ke-Wei is absolutely right and mentions an important aspect. The 'phosphorylation-signal' can be very short-lived and is prone to attacks by phosphatases. To prevent these attacks fast handling to harvest cells and the addition of phosphatase-inhibitors such as vanadate to media/buffers are useful.