Although your 5 Rep Max (5-RM) is the weight you can lift 5 times in a row, but not 6, 5-RMs are not tested by doing sets of 6 with increasing weights until you can only finish 5.
Instead, after warming up, do sets of 5 with increasing weights until you get to a weight that you cannot make all 5 lifts. You then take your last successful set of 5 as your 5-RM. The same can be applied for 1-RMs, 3-RMs, 10-RMs, and any others you may be interested in.
If you have no idea what your RM should be for a lift, begin conservatively. After each set of 5, decide how much weight you will add based on how easy the set felt – if it was really easy, add a lot, if it was more difficult, add less. If you are unsure, play it safe and add less.
If you know what your RM or what it should be based on previous lifts, aim for your third set to be your expected RM. That way you have warmed up sufficiently from the first two attempts to help increase your chances of making a heavier lift, and you have gotten to your previous RM without prematurely fatiguing yourself.
The goal is to find your RM within 3 to 7 sets. Finding it in fewer than 3 sets may not give you enough of a chance to warm up so you may fail at a lighter weight than you can actually lift. Taking more than 7 sets will likely fatigue the muscles to the point that you will underestimate your true RM.
Be sure you ramp up to your first RM attempt with warm-up sets to prime your nervous system and the muscles involved in the lift. Use jumps of 50-100 lbs. initially if you are going into the hundreds, and bring the warm-up weights closer together as you approach your first attempt.
E.g. For a 225 first attempt, warm-up weights could be 45, 95, 135, 185, 205. If you think that doing those warm up sets will fatigue you too much for your first heavy attempt, then you may not be ready to test that heavy weight and should work on your strength endurance. Alternatively, you could drop the number of reps performed as the warm-up weight increases to prepare the muscles for heavier weights without performing too much work.