You need to visit their website and check out their spread page, also collect information about their execution policy and the sources of their price feeds.
Best is to go to myfxbook and sort the list. I agree with most here, floating spreads would be a better bet. go for a broker like fxview, commission cost is low as well as spreads.
when you have lack of knowledge i think no trading spreads even low can help , to bring profit consistently need powerful money management first of all
Draw comparisons between low spreads brokers you finalize on the basis of their commissions. This was a differentiator for me when I finally went with hankotrade.