Start with the frame
The frame decides the question more than anything else. A solid wood, vinyl, or fibreglass frame with a faulty lock, a broken balance, or worn weatherstripping is a repair job. A frame with rot, warping, or chronic water damage is a replacement job — patching it buys time, not performance.
Check each window individually. It is common for a house to need three replacements and five repairs in the same project.
Fogged glass is a glass problem
Condensation between the panes means the sealed unit has failed. The frame may be perfectly fine. In most cases a glazier can replace the sealed unit while keeping the existing frame, at a fraction of replacement cost.
Cracked or broken glass is similarly straightforward. Glass replacement is one of the clearest cases where repair wins.
When replacement is the honest answer
Replacement makes sense when several problems stack up: the frame is deteriorating, the glass is failing, the hardware is obsolete, and the window was inefficient to begin with. At that point, the cost of repeated repairs approaches the cost of a new unit that will perform better and carry a warranty.
Also consider replacement when you are renovating anyway. New windows installed during a larger renovation cost less per unit in disruption and often in price.
Get both opinions
Ask a repair-oriented company and a replacement-oriented company to look at the same windows. If both independently recommend the same course of action, you can proceed with confidence. If they disagree, the disagreement itself is useful — ask each to explain why, and the better reasoning usually becomes clear.