That the phone can take an eSIM at all
Look in the phone settings for an option to add a cellular or mobile plan — wording varies by manufacturer and by Android skin. If there is no such option, the handset has no eSIM and no purchase can add one. Check the manufacturer spec sheet for your exact model number rather than the marketing name: the same model name is sometimes sold in a variant without eSIM in some markets.
That the phone is not locked to a carrier
A handset sold subsidised by an operator can be locked, and a locked phone refuses a profile from anyone else. This is separate from eSIM support and it is the single most common reason an install fails after payment. Ask your operator to confirm the phone is unlocked before you buy anything.
Whether the plan is data-only
A plan sold as data-only carries no phone number: no incoming calls, and no SMS — including the one-time codes banks send. The product page will say which it is, and it is worth reading rather than assuming. Keep your home SIM enabled for SMS if you rely on those, and make sure calling apps are set up before you leave, because installing one needs data you may not have yet.
Which countries the plan covers, and only those
Plans are sold as local (one country), regional (a defined group) or global. A local plan stops working at the border, and a regional plan covers the list it names and nothing else — a neighbouring country is not implied. Read the country list against your actual itinerary, including layovers long enough to leave the airport.
What starts the validity clock
A plan with a validity window may count from purchase, from installation, or from the moment the profile first attaches to a network. Those are three very different things for a trip booked weeks ahead, and it is the difference between a plan that expires mid-holiday and one that does not. The answer is in the plan terms; if it is not stated, treat that as a reason to look elsewhere.
Whether you can top up
Running out of data is normal; being unable to add more without repeating the whole install is not. Check whether the plan supports a top-up that keeps the same profile, and whether a top-up extends the validity window or only the allowance.
Which local networks it uses
A travel eSIM provider does not own a network. It resells access to local operators, and the operator it puts you on decides your real speed and whether you have signal indoors, in a rural valley, or on a train. Where the provider publishes the partner networks per country, compare them against local coverage maps for the places you are actually going.
Whether tethering is allowed
Sharing the connection to a laptop or a travel companion is permitted on some plans and blocked on others. If a hotspot is part of your plan for the trip, confirm it before buying rather than discovering it in a café.