Electrical services age differently than the rest of a house. A roof announces its retirement. A panel just quietly becomes the reason you can't have the things you want.
Start with what you have. Open the panel door and look at the main breaker: the number on it is your service size. If it says 60 or 100, or if there is no main breaker because you have screw-in fuses, you're working with a service that was sized for a household with far fewer electrical demands than yours.
The signs are usually behavioral before they're dramatic. Lights dip when the fridge compressor or the well pump kicks in. You've learned not to run two particular appliances at once. The panel is full, with tandem breakers doubled up in slots to buy space. There's no room for the circuit you actually want to add.
Then there are the plans that force the issue. A heat pump, an induction range, a hot tub, a finished basement, or an EV charger each add substantial continuous load. This is why we run a load calculation before quoting an EV charger rather than after: it's better to learn about the constraint during the estimate than on installation day.
Some panels are worth replacing regardless of capacity. Federal Pacific Stab-Lok and Zinsco panels have well-documented histories of breakers that fail to trip under fault conditions. If you have one, capacity isn't the argument, reliability is.
A proper upgrade is more than a bigger box. It includes coordinating with the utility at the meter and service entrance, running a correctly sized service conductor, driving and bonding a grounding electrode to current code, moving every existing circuit onto the new panel, and labeling the directory so it's readable. It's permitted work, and it gets inspected. Ask for that paperwork; it stays with the house and it matters at resale.
One last note for multi-family owners: if your building's circuits cross between units, an upgrade is the right moment to separate them properly. It costs less to do while the panel is already open than to revisit later, and it removes a genuine source of disputes between tenants.