Short of buying a Spectrum Analyzer, here are your other alternatives:
1. Ask your neighbors if they have OTA? Since you're in Baltimore City (BTW, my wife is originally from Baltimore City and always corrects me when I call it Baltimore!) then signal strength will not be an issue. I receive digital OTA from Baltimore City (90+ miles) much of the time between Mar-Oct. Your issues will most likely be affected by multipath. Anyway, if your neighbors are receiving a reliable signal then the odds are pretty good that you will too.
2. You may very well be able to receive your digital locals by using an inexpensive indoor antenna like the Silver Sensor. Assuming you don't already have an OTA tuner, you may wish to purchase an open box Samsung OTA tuner at Best Buy and later return it if you're comfortable with this idea.
3. In my opinion, have VOOM install the system and, as the VOOM guy stated, do not accept service if he cannot get OTA to work to your level of satisfaction. There is absolutely nothing wrong with doing this since you are making a sincere attempt subscribe to the VOOM service.
4. Also, if this is an option and you can afford to do so...you may look at subscribing to a 'Lifeline' cable subscription, which typically includes your local channels and a few public access channels. The lifeline package typically isn't advertised and will vary in price depending on your cable provider. I've seen then as cheap as $9.95 and as expensive at $35; it's $19.95 in my area.
Good Luck...