When working proof exercises from a textbook with no solutions manual, how do you know when your proof is sound/acceptable? 
When working proof exercises from a textbook with no solutions manual, how do you know when your proof is sound/acceptable?

Often times I "feel" as if I can write a proof to an exercise but most of those times I do not feel confident that the proof that I am thinking of is good enough or even correct at all.  I can sort of think a proof in my head, but am not confident this is a correct proof.
Any input would be appreciated. Thanks.

Use a computer with automated proof checking software, also called a proof assistant or interactive theorem prover. Typically you will need to write your proof in a special, machine readable format (be careful for translation/copy errors), but past that point this field is well studied and computer-based proof checking is generally reliable. Wikipedia has a comparison table of different software for this purpose: http://en.wikipedia.org/wiki/Proof_assistant . (If this is preferable to, easier than, or even faster than hand-verifying your proof step-by-step is another question.)
