In the most common time signature 4/4, each bar is divided into four sections, we call these beats. The first of these four beats is a strong beat, the second is a weak beat, the third is a medium beat and the fourth is a weak beat.
A strong beat is a beat which has a strong accent relative to the other beats. Some signs of a strong accented beat are loudness, longer length and higher pitch. Which are also the ways you put stress on a word when speaking.
So, to figure out the tempo of the beat you're working with just try to hear where the strong beats are and how often. If you can't distinguish where they are and you've listened very carefully I would just pick whatever tempo suits you and work from that.