How can you confirm at the outlet with a meter if it truly is a 20A dedicated line and has not just had the outlet changed from 15A?

Any such attempt with a meter would just destroy the meter. Why not just find out which breaker/fuse controls the circuit and see what the breaker/fuse is rated at. Also find out what the wire size is. For a 20 AMP circuit you MUST have 12 gauge wire. There are other requirements but those two seem to be about the most significant. As has been stated, a meter won't help. The breaker prevents excessive load to be placed on the wiring so any 20 amp circuit would be limited by the breaker and there would be no other way to determine if a circuit is 15 Amp or 20 Amp other than to check at the breaker panel. Also, if someone just put a 20 amp breaker on a 14 gauge wire you have a serious fire hazard. As the above post suggests, have a professional check this out for you, it's too dangerous to make assumptions. <><><> Your description of "20A dedicated line" makes me think that you are needing a circuit for a heavy power-using-device that needs it own circuit (without any other receptacles or devices). If that is the case, then a meter can be used (with difficulty) to determine if any other outlets are connected to the same circuit. First turn off the power on the receptacle. Then check to see if any other receptacles are also dead. If so, they are probably on the same circuit and your receptacle is not a dedicated supply. <><><> Another method is to use a circuit tracer (available at Lowe's and other hardware and electrical supply stores). Plug the transmitter into the questionable receptacle, then use the receiver to check other receptacles to see if they are on the same circuit. Receptacles on the same circuit will cause a continuous fast beep (or other signal depending on model). You can also trace the circuit to the circuit breaker panel to find which breaker it is connected. It should be a 20-Ampere-trip breaker to make full use of a dedicated 20 A receptacle. <><><> Borrow an amp clamp and find the breaker in question. Clamp it around the neutral for that circuit (This takes a friend helping). Plug in a significant load like a hair dryer and have the friend turn in on and off while you watch the meter. It should read zero, then 8-10 amps then zero, etc. Plug loads in outlets that you think the contractor absconded the circuit from. The amps from those outlets should never show up on the amp clamp. Make sure the wire is 12 awg. A 12 awg wire will not push in the stabs of a 15 amp receptacle. SAFETY ADVICE

Before you do any work yourself,

on household electricity supplies, equipment or appliances,

always turn off the power

at the breaker box/fuse panel BEFORE you attempt to do any work AND

always use an electrician's test meter having metal-tipped probes

(not a simple proximity voltage indicator)

to ensure the circuit is, in fact, de-energized. IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB