For another measure of network quality, turn to independent tests on the carriers. PCMag, for example, recently published results on the fastest and most reliable networks. AT&T and Verizon were neck and neck, T-Mobile lagged only slightly behind Verizon and Sprint was a distant No. 4.

Word of mouth goes a long way. If you are thinking about switching to a carrier, check with friends in the neighborhood who subscribe to that network first for an assessment on speed and reliability. The more data you have, the better.

Measure your data consumption

How much data do we really need? This is a tough question because most of us don’t know how many bytes we consume when we stream a song, load a photo on social media or send a message.

Everyone’s data use varies, so you will have to look at your monthly data consumption and calculate an average. On your carrier’s website, log in to your account and look up your data use summaries.

You can also check data use directly from your mobile device. Here’s how to do that for each carrier:

For AT&T subscribers, place a phone call to *DATA# (*3282#). AT&T will send a text message showing the amount of megabytes you have used out of your monthly allotment.

Verizon subscribers can place a call to #DATA (#3282). Verizon will send a text message containing a report of data use.

Sprint customers can send a text containing the word “Usage” to the number 1311. Sprint will respond with a message containing a data summary.

T-Mobile subscribers can place a call to #WEB# (#932#). T-Mobile will respond with a data summary.

Here’s where things get tricky: If you plan to buy a new phone that is much faster than your current one, you should err on the side of buying more data. For example, if you have a five-year-old iPhone and use three gigabytes of data a month, and you intend to buy a new iPhone this fall, consider upgrading to a plan with five gigabytes. You will most likely use more data with a faster device.

Be wary of the fine print

Carriers offer plenty of attractive perks, like free international roaming packages and so-called unlimited data, to persuade you to switch to their networks. Don’t let these perks drive your decision because there are always caveats, Mr. Toikka said.

For example, T-Mobile offers free international roaming, which lets you use a cellular data connection in more than 200 countries at no extra charge. The caveat is that free roaming only gets you access to slow wireless speeds, and you have to pay extra if you want faster internet access. (If you want to travel with your phone, you are probably better off using a foreign SIM card, as I detailed in a previous guide.)