Just do a set to failure on any exercise, then enter in how much weight you did and how many reps you finished into the form below.
Never, under any circumstance should you attempt a one rep max alone. We would recommend taking two spotters (one for each side of the bar) at least. Safety should be kept in mind at all times.
This method is based on a formula from "Maximum based on reps" by J. Landers, published in 1985 in an NSCA journal.