How about referring to "as built" general arrangement drawings of previously-constructed panels?

Check them for what? Lice? Mold? Color retention? Exothermic Delinearization from gamma radiation exposure on a full moon?

if you are asking how to check busbar size ,then Use a measurement tape.

if u are asking how to size the busbar, here is an extract from copper institute

Design Requirements

The current-carrying capacity of a busbar is usually determined by the maximum temperature at which the bar is permitted to operate, as defined by national and international standards such as British Standard BS 159, American Standard ANSI C37.20, etc. These standards give maximum temperature rises as well as maximum ambient temperatures.BS 159 stipulates a maximum temperature rise of 50°C above a 24 hour mean ambient temperature of up to 35°C, and a peak ambient temperature of 40°C.ANSI C37.20 alternatively permits a temperature rise of 65°C above a maximum ambient of 40°C, provided that silver-plated (or acceptable alternative) bolted terminations are used. If not, a temperature rise of 30°C is allowed.These upper temperature limits have been chosen because at higher maximum operating temperatures the rate of surface oxidation in air of conductor materials increases rapidly and may give rise in the long term to excessive local heating at joints and contacts. This temperature limit is much more important for aluminium than copper because it oxidises very much more readily than copper. In practise these limitations on temperature rise may be relaxed for copper busbars if suitable insulation materials are used. A nominal rise of 60°C or more above an ambient of 40°C is allowed by BS EN 60439-1:1994 provided that suitable precautions are taken. BS EN 60439-1:1994 (equivalent to IEC 439) states that the temperature rise of busbars and conductors is limited by the mechanical strength of the busbar material, the effect on adjacent equipment, the permissible temperature rise of insulating materials in contact with the bars, and the effect on apparatus connected to the busbars.

The rating of a busbar must also take account of the mechanical stresses set up due to expansion, short-circuit currents and associated inter-phase forces. In some busbar systems consideration must also be given to the capitalised cost of the heat generated by the effective ohmic resistance and current (I2R) which leads to an optimised design using Kelvin's Law of Maximum Economy. This law states that 'the cost of lost energy plus that of interest and amortisation on initial cost of the busbars (less allowance for scrap) should not be allowed to exceed a minimum value'. Where the interest, amortisation and scrap values are not known, an alternative method is to minimise the total manufacturing costs plus the cost of lost energy

also check this website

Busbars

In LV Panel board, Bus bar sizing will be done as follows:

1. Bus bar shall be High grade Electrolytic copper
2. It shall be shape of more rectangular than square
3. It's current density shall be 1sqmm=1.5 to 1.6Amps or 1.5 to 1.6Ams per mm²

The above is limited to Bus bar sizing only.

You mean .... to check the bus bar sizes in LV panels? Then simply take a measurement tape & physically measure (of course, with the power supply to the panel totally cut-off). Then compare this with the GA Drawings provided by the panel manufacturer.

Else... if you want to check the adequacy of the bus bars with respect to its rated current & short circuit withstand parameters, then you have to attend a detailed training.