How does one know whether powder is still good?

I have two small batches of leftover MP200 - one I bought new and another I got from a fellow shooter. If I throw them together I might have one third or half a tin - enough to reload quite a few cartridges for practice.

Trouble is that their colour and smell differ very slightly. I burnt some of it in a spoon and it ignited well.

Any suggestions and ideas appreciated!

I don't know any definitive test but have never had an issue with old Somchem powder. If the tin is sound, the 'stopper' is in and there's no rust on the inside I use it. I'm currently finishing up a tin of 1980's vintage MS in the Musgrave packaging. Chronied a couple and they were within shot to shot variation of expectation. Interestingly this stuff is impossible to tell apart from the most recent MS that I bought approx 3 years back. 

To continue the question:

How does one test an older/unknown batch of powder?

It can only deteriorate, therefore my only concern should be a squib. Unlikely, but noteworthy.

Should one load a few shots of medium or hotter loads to prevent a squib, rather than a very low load which could end in a kaboom?

My standard range loads for shotgun and pistol are in the middle of the somchem tables so I start there with 5 to 10 cartridges and chrony them. From tins of MS and MP in good condition as described above I have never found a variation in velocity that would be out of place in a string of shots from a recently supplied tin. I may just be lucky in that but I suspect that appropriately stored powder lasts a very long time given that I've used stuff approaching 40 years old with successful results. I'm currently using a batch of 12g slugs loaded with 30 plus year old MP and Swartklip primers in the tins sealed with tape. I have no idea how old those are but they certainly aren't 'fresh'. 

So the propellant is old - we know this to be safe (usually)

If you meaning the cartridges are old, be careful - hangfires - squibs and blocked barrels happen. I have not found shotshells to age well.

I would load a few round of each to the same powder level and check velocity.

This will also tell you if in fact they are the same powder.

If the velocities are similar, mix them well and load up.

If not, just use separately.

Shotshells do not store well in my world

I would load a few round of each to the same powder level and check velocity.

This will also tell you if in fact they are the same powder.

If the velocities are similar, mix them well and load up.

If not, just use separately.

This will do, thanks.

So the propellant is old - we know this to be safe (usually)

If you meaning the cartridges are old, be careful - hangfires - squibs and blocked barrels happen. I have not found shotshells to age well.

Not cartridges, reloads with Swartklip primers and Musgrave branded MS200.