The above discussion is a good one. I would also start with the basics. I would use the IRWIN FOB (functional observational battery) to begin to determine if there are any toxic effects or side effects. This test looks at things like homecage activity, muscle tone, lacrimation, ptosis, grip strength, piloerection, righting reflex, etc. This can give you a good basic idea if there is a side effect profile with your drug. In addition, you will want to perform post-mortem tissue, blood and organ analysis looking for damage, apoptosis, etc. As someone mentioned on the linked thread above, these things must be performed at each dose, route of administration, and dosing regimen.
Here are a few links for the FOB testing:
Article Primary Observation (Irwin) Test in Rodents for Assessing Ac...
Article Functional Assays for Neurotoxicity Testing
Article CNS Adverse Effects: From Functional Observation Battery/Irw...
This is a very common question, but the scope of answering is very wide. Try narrowing down your question as to what kind of toxicity you really want to focus upon. I am attaching few papers, so that you can gain some idea on drug toxicity testing. Will be happy to reply if you have more specific queries later.