There are dozens if not hundreds of Trekking agencies based in Kathmandu and Pokhara. Many of them supply a decent service, and picking a possible good one out of the lot is very much akin to choosing a potentially good restaurant. Peek in, have a chat, see if they are busy with clients, check what prices and services they are offering, get a feel of their degree of honesty and check recommendations. This is a buyer's market.

Having narrowed your choice to a few agencies, interview the guides they have to offer. This is the most critical step in organising the trek; once you have chosen your man (or woman) and are headed for the hills, you are stuck with him/her for the next weeks. If discord sets, it is guaranteed to spoil your walk, so choose wisely. We chose a guide and an agency that we thought we could trust, and it worked out ok for us. It is not necessarily so for everyone. 

You will find here advice on Guide selection and Costs