Evaluating the quality of photomasks
The simplest way to evaluate the mask is just to inspect it visually, as you say you have already done. Be sure to take a look at the critical (ie smallest) features to ensure the mask looks how you expect. With a good light microscope with high NA objective, you can get sub-micron resolution on optical images.

Another things you can check if you really want to dive deep into evaluating the quality of the mask is to measure the consistency of the height of your features. If the features on the mask do not press against the resist evenly across the whole wafer, some areas on the wafer might get exposed unevenly. You could look at this with a profilometer or even SEM if you want to really get into the weeds.

That being said, testing it out in the actual process is the best way to examine the quality. In my experience, I usually have to expose a couple test wafers to optimize the exposure time anyway, so this provides a built-in test of the mask. Having an exposure grid with different sized features can make this process a little easier.