None of the things previously mentioned will be much good unless the audience or trainees have confidence in the speaker. This can be achieved in a variety of ways. The first thing to say is that, under no circumstances, should the speaker resort to boasting. A comment like "I've been doing this for years and I know what I'm talking about", is guaranteed to generate resentment and a loss of confidence in the speaker/instructor.
The methods used need to be much more subtle and some of them are shown above.
EXPERIENCE. You can refer back to previous courses conducted. Maybe someone asks a question. You say, "Ah the last course I did this came up and the answer is .......etc". Maybe it's not quite true but who's to know. It gives the impression that you have done this before.
CERTIFICATES. Don't simply get out all your certificates and show them. Instead you could show a blank forklift operator certificate and say "this is what you get after you pass your test" Then you get yours and your instructor's certificate and tell trainees "this is what it looks like when it's all filled in and this is my instructor certificate. Maybe you will get one of these someday"
TRAVEL. Maybe you have conducted training courses at other branches of the company? Refer to them. "when I was in London I came across...... etc"
SPECIAL KNOWLEDGE from research. "I was checking up about this on line last week etc....."
You won't be able to get all the above things in one session but gently dropping the odd thing in will greatly increase the audience/trainee's appreciation of the speaker/instructor.