I want to make sure that my agreement is considered fair for both my spouse and me. How can I be assured?

You could put in a provision stating your intentions are to be fair. Such a clause is quite common, both in separation agreements and other types of contracts.

A typical clause in a separation agreement reads as follows:

The wife hereby acknowledges that the foregoing provisions for her spousal support are satisfactory, reasonable, and sufficient provisions for her. The parties each waive all other rights, if any, to seek or obtain spousal support from the other, except as otherwise contained herein.