If you and another player have the same level base how can you tell if you can beat them before attacking?

Same lvl base is not guarantee you have the same power. It depends on tech research and building upgrades. Camp your strongest APC nearby and wait for the attack to commence. check his power and if you are better port very close to him and attack him with all your apc's and continue till he is ported away.

Attackers seem to have an advantage in fights but it is a good idea to check their power level first. Tap the city in the map and then tap the info button to see their power level. If it is much higher than yours then they likely have a lot more troops.

You can also scout their city before attacking although this will alert them if they are online and may cause them to shield or trigger an attack back at you regardless of whether you want to fight.