Some symptoms and behaviours may be strain specific, but in general most mice with brain tumours should present with weight loss and  arched back. Sometimes paralysis of legs (they won't fully extend leg when walking, or drag it), less active, grooming more than usual and  repetitive behaviour. If you know the normal behaviour of your mouse strain its easier to pick out aberrant behaviour but I'd recommend monitoring for weight loss on a regular basis.

Thank you very much Pawel. your suggestion was very helpful. Do you have any reference for some of the behaviours you suggested above?

Sure. Here is a quote from Sharma et al, Cancer Res, 2014 : http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3755354/

"Orthotopic tumor implantation

The procedure for brain tumor implantation was similar to that reported previously (24). Briefly, nude mice (Male NIH Swiss nude mice, nu/nu, 5–7 weeks old) were anesthetized and secured in a stereotactic apparatus. A suspension of glioma cells (U87/EGFRvIII) was prepared fresh from culture (105 cells/μL in phosphate-buffered saline (PBS)) and injected (5 μL) into the caudate putamen. After 1 week, mice were monitored once a day for symptoms related to tumor growth that included an arched back, unsteady gait and loss of body weight. Mice were entered into the PK/PD studies when they showed a total body weight loss of 10% or more on two consecutive days."

Thank you so much.