You can use a 3 wire feeder to supply a separate building, if...

• The installation was in compliance with a previous edition of National Electrical Code (existing premises wiring).
• An equipment grounding conductor is not run with the supply to the structure.
• There are no continuous metallic paths bonded to the grounding system in each structure (bonded water, or gas piping, other conduit, etc.).
• Ground-fault protection of equipment has not been installed on the supply side of the feeders.

National Electrical Code 2014

Article 250 Grounding and Bonding

II. System Grounding

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).

(B) Grounded Systems.

(1) Supplied by a Feeder or Branch Circuit. An equipment grounding conductor as described in 250.118 shall be run with the supply conductors and be connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

Exception No 1: For installations made in compliance with previous editions of this Code that permitted such connection, the grounded conductor run with the supply to the building or structure shall be permitted to serve as the ground-fault return path if all of the following requirements continue to be met:

(1) An equipment grounding conductor is not run with the supply to the building or structure.

(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.

(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).

If the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:

(1) That required by 220.61

(2) That required by 250.122

Changing from a 30A breaker to a 50A breaker can only be done, if you also change the wires to 6 AWG. In which case you'll have to follow current codes, and install 6/3 with ground. Breakers (and fuses) are always sized to protect the wire connected to them, so you can't change the breaker size without also changing the wire size (unless you're going down e.g. 50A to 30A).

However, depending on what you're doing, you may not have to change the breaker at all. If the planned circuits in the structure are not going to be fully loaded, you may well be able to supply the subpanel with a 30A breaker. Just because the subpanel has 50 amperes worth of overcurrent protection, does not mean the supply breaker has to be 50A. Whether or not you actually need a 50A breaker on the supply, depends entirely on what the subpanel will be powering.

TL;DR: There is not a 100% safe or 0-liability option to intentionally pull more current through a circuit than allowed by applicable codes.

The cheapest design I can think of is a non-contact clamp ammeter that graphs current clamped to a piece of thick gauge wire. Turn off power to the circuit and then short the thick gauge wire between the outlet terminals. Flip the breaker back on to let it trip and then look at the max current pulled. However, this seems like a huge liability should the circuit pull significantly more current than its rated capacity because it could overheat and either immediately start a fire or pose a potential fire hazard in the future. I personally would never do this and never recommend it since it seems like the test poses too much of the hazard you would be trying to prevent.

The next design I can think of creating also uses a clamp ammeter that graphs current clamped around a wire to a test load. You would create/buy a series of test loads sized to pull just a little more current than the circuit's wire gauge should allow if the circuit is wired correctly and operating normally (e.g. the circuit breaker isn't fault), but not so much as to pose a significant fire hazard over a short period of time. As an example: Use a 5 ohm power resistor inside of a high-heat insulated enclosure to limit current to 24 amps through a 120V / 20A circuit. Similar to the first design, you would turn off power to the circuit, plug the test load into the outlet, restore power, and then remove power if the over-current protection device does not turn off power to the circuit within a second. However, I don't have the data to support that this is 100% safe and it still seems like you could potentially be opening yourself up to future liability as an electrician by performing a test this way. I therefore could not recommend either of these methods or any method that intentionally pulls more current through a circuit than allowed by the applicable codes because there is always some room for liability if you accidentally damage the wiring by pulling more current through the circuit than allowed.