Does anyone know if Batteries Plus or some other place can test the lithium ion battery pack for my e-bike? I know most all parts stores can test car batteries real quick. I would like to know if I have usable ah left.

One option is to do this yourself. You will need a simple inexpensive Watt Meter and a resistance load (kettles, toasters, etc.). The procedure is:
1. Fully charge the pack
2. Connect the input side of the watt meter to the discharge port of the battery
3. Connect the load to the output side of the watt meter
4. Let it run till the battery BMS shuts things down and see how many Ah have been used.

The results will be more meaningful if you can find a load that will generate a meaningful discharge current (10 amps or so)

Pretty sure an electric light bulb is good for "discharge".

Yes..but wouldn't a 100 watt bulb draw only about 0.5 amps at 50V? Quite a slow rate compared to what real riding would draw plus for a 10Ah battery it would take in the neighborhood of 20 hrs for full discharge. A 1500 watt (rated at 120V) load should draw about 7 to 8 amps at 50V.

I have seen people using this analyzer: West Mountain Radio - CBA IV - Computerized Battery Analyzer

Only 100watts continuous. The numbers for instantaneous power are fairly high though. I think people put bigger power stages on them for more continuous power dissipation

Wattmeters are also available for a little more from US vendors on ebay or amazon. .I have a coup[le of the latter type. They are good for telling max current and the AH used.

In my opinion, the working capacity of your battery in AH is a useful indicator of its health, If that value is close to the original rating, the battery is in good condition. If it's only half, well, you're getting half the range and you already know it's a sad battery, but a wattmeter confirms it.

Ride your bike til the battery is almost flat with a wattmeter hooked up, and look at the AH that were used. If you ride til it's really flat, the wattmeter loses its data. Or you can put it on the charger line and measure how many AH are needed to recharge it. You can also use a light bulb for a load, instead of riding. SOme users talk of using hair dryers. Whether discharging or charging, power is conserved. You should get the same number.

Again, if the battery shuits off, the wattmeters lose the data. Ive ridden on and on to discharge the battery only to lose the data. They do have connector for an external battery. A 9 V transistor radio (who remembers those) will power it. Then you can let the battery go flat.

The battery shops mentioned by the OP are probably only set up to do load tests on 12 and 24V batteries. Maybe they can do a 36.

I also have a couple of those wattmeters. I was a bit surprised to find that the last one I bought actually retains the data when power is lost. As you suggest, this can be quote useful when doing a capacity discharge test.
Here's the link to the one I bought (the internal shunt version).

DC Combo LCD Panel Meter Car Solar System Battery Voltage Current Power Monitor