We all have an idea of how much we want to lift going into the gym or a meet. However, how much you want to lift versus how much you can lift on any given day will vary. I have been using the same technique for years to pick my lifts as well as attempts for others.
My method is not rocket science, but it does require you to be honest with yourself. After a lift when deciding to go for another, a number will pop into my head. I'll immediately get a feeling of excitement or doubt. If I have doubt, I drop the number. If I'm excited, I go for it.
When another lifter asks me what to take, I ask them "What do you think?" at which time they'll throw out a number. To that, I ask "Do you think you can hit that?" If I don't get an immediate answer of something like "Hell yeah!" If there's any hesitation I drop the number and tell them they should go for that. 99 percent of the time the lifter will say "That will be easy!" My response, "Good, then show me.
The mind plays a much bigger role in strength training and competition than most know. I'm still always fascinated by how few competitors utilize sports psychology. So when picking lifts for yourself or others, if there is a question, there's no question.