Yep, I'd say regular expressions are your friend here:
var match = Regex.Match(input, @"[0-9.]+e[-+][0-9]+");
Or you can prevent matching multiple decimal points with the following (the last one will be treated as the "proper" one):
@"\b[0-9]+(.[0-9]+)e[-+][0-9]+\b"
Edit: Here's a more complete one that will allow optional exponents and will also allow for the decimal point to be at the start of the number:
@"[\d]*\.?[\d]+(e[-+][\d]+)?"