george is right that the correct way to do this is with a vacuum gauge, etc. But luckily you have a gas-engine car, where the vacuum for the modulator is a simple engine-vacuum line. If it was a diesel you'd also have to deal with the part that is "creating" the vacuum that is fed to the modulator.

You can at least do a bit of debugging of your symptoms by pulling the vacuum line off of the modulator (should plug the end of the dangling line), and then verifying that you have firm/hard shifts. (No vacuum is equivalent to hard acceleration with firm/hard shifting - high vacuum is equivalent to slow acceleration or letting up on the gas pedal, with smoother/softer shifts.)

If you don't see a change it is possibly an indicator of a failed or leaky modulator. Some people feel that it is possible to adjust the modulator by "feel" - I'm sure if you search the forum you will find a post that talks about turning the little key that is under the rubber dust cap of the modulator. But it is a better idea to have the modulator adjusted by someone with the appropriate gauges, etc.