the best marker to study muscle differentiation and then myotube formation is Myosin Heavy Chain (MF20 antibody clone) works perfectly in IF and WB, beside you can use also Desmin is commonly accepted also if personaly I do not like so mutch. Furthermore there are trascription factor (MRF) likewise MyoD and Myogenin that works quite well in IF and WB.
I strongly agree with Cesare. Also Myosin Skeletal Fast (Sigma) works perfect for IF and Wblot analyses. Desmin is not one of my favorite marker. Myogenic works almost perfect, peaking at day 3-4 after differentiation media induction.