Electrical Motor

If the motor is in place and running, all you need to do is measure the voltage and the amperage which will give you the wattage which will give the hp, around 745 watts per hp....If it's just sitting alone with no history, it's scrap...RPM can be determined with an rpm counter...

http://www.electron.it/pdf/catalogue/LEAFLET%20A47%20SERIES%20(INSTRUMENTS%20FOR%20ELECTRICAL%20MACHINES).pdf

I don't agree that this post is totally Off Topic, just slightly inaccurate for the following reason:

There is no way to easily DETERMINE the mechanical output capability of an unmarked motor, because as mentioned, all you can do is measure RLA, shaft torque etc., but that does not provide you with design limits. But I think that measuring LRA is actually a decent way of ESTIMATING what the designed power rating might be.

However if you don't know what the design VOLTAGE is either, then getting LRA would be problematic in that if the motor was designed for a lower or higher voltage than what you apply to test it, the results would be meaningless.

In some countries there is only one LV (400v)for 3ph motors. I was under the impression that the OP is from such a country. In Saudi arabia I have come across 3 standard voltages for LV namely 220,380 and 480. There sump pumps are normally with 220V single phase motors. In our company a pump came for repairs and after rewinding the technician gave 220V and motor burnt. Upon checking we found the rated voltage was 110V. The technician said the name plate was not clear and he was not given a magnifying glass to check the nameplate.If you know how to solve the problem why don't you come out with your solution.

Rpm can be measured by a rpm counter.Determining HP is difficult.You have to do atemp ries test either as is done in a factory or if u have some loading device it can be done.