How do i know if my employer is paying me correctly?

1) Your employer can only change your rate or how you are paid "going forward" or on a prospective basis: that is, forward from when you are told of the change. They cannot change it retroactively or without notice. They may change it at will, however, unless you have a written employment contract limiting their ability to change your pay.
2) You could be paid on a salary basis: if so, your weekly salary, which should be the same amount, regardless of hours worked, week to week, is your total pay for all time spent working.
3) If paid hourly, you are only paid for when you work--but work includes driving to a client, waiting for a client, etc. Basically, anything you do because the employer told you to is "work" and an hourly employee would be paid for it.
4) Whether paid hourly or salary, for the job duties or responsibilities you describe, you should be paid an overtime premium when working more than 40 hours in a week (all time past 40 hours should receive a premium).
5) You get to fill out, etc. your W4; it's not for the employer to make the decisions in regard to it for you.
Based on what you write, your employer may be violating the wage and hour laws, if it is making changes to your pay without notice and not paying you overtime when you work, evidently, 50 - 84 hours per week. You may wish to contact the Dept. of Labor to file a a complaint.