Te and Se can look similar as they are both action oriented and high energy, especially when dominant, and they both have a very "get-it-done" mentality.
The main difference as I see it, is that dom Te is explicitly goal oriented and concerned with creating order, whereas dom Se is very dynamic and enterprising, and tends to thrive in chaos. Te plans linearly carries through to execution, whereas Se tends to be more along the lines of "act first, think later."
There are some cases in which it becomes more difficult to differentiate Te and Se.
For example, in the ISTP, the dom Ti + aux Se can sometimes create a restrained and structured quality reminiscent of Te. (For example, consider Erwin Rommel, an ISTP that people often confuse for an xNTJ. His motto was "erst wägen denn wagen: first wager, then risk", which is similar to the pro/con oriented thinking of Te types) and in the ENTJ/ESFP, the Te and Se can be somewhat blended.