Start at the door: one exit, and it seals
Proofing a room is mostly a plumbing job for gaps, and the biggest gap in almost every room is the one under the door. Fit a proper door sweep or brush excluder strip so the bottom of the door meets the floor along its whole width, and test it the way a snake would: slide a thin card along the gap and find anywhere it passes through. While the door is open, also check the hinge side and the frame for warped sections. A room whose door seals is a room an escapee can be searched in; a room whose door leaks is just a corridor to the rest of the house.
Get down to floor level and hunt
Owners look for gaps at eye level; reptiles find them at skirting level. Get on the floor with a torch and work the perimeter slowly. The usual suspects are gaps where skirting boards meet uneven floors, holes drilled for radiator pipes and cables, the open backs of kitchen units and the void behind a washing machine or fridge, and floorboard seams in older houses. Seal pipe and cable holes with escutcheon plates, filler or tightly packed mesh, and fit fine mesh covers over air bricks and vents, choosing a mesh small enough for the smallest animal that will ever use the room.
Match the proofing to the animal in the room
A tortoise is a ground-level escape artist: it cannot climb, but it can bulldoze, dig at carpet edges and wedge itself behind furniture, so its proofing means barriers it cannot push through and nothing small or swallowable left in reach. A snake is the opposite problem. It climbs as readily as it crawls, so proofing for a snake extends upward: gaps around window frames, the tops of doors, curtain rails that lead to vents, and any pipe that runs floor to ceiling is a ladder. Small lizards and frogs defeat coarse mesh and wide excluders alike, so scale every seal to the smallest body, not the average one.
The open-and-shut ritual that keeps it proof
Even a well-sealed room fails on habits. Enclosures open only when the room door is shut. Anyone entering during free-roam or cleaning time knocks, opens slowly and checks the floor before stepping in, because a lizard basking against the door is invisible until the door moves. Before the door opens at the end of a session there is a head count: the animal is in a hand, in a holding tub or visibly back in its enclosure. It takes ten seconds and it is the step that makes the seals worth fitting.
Because a room is only proofed while everybody treats it that way, pin the rules to the pet's shared details in KinsLe's Family Sharing, so family members, older children and anyone house-sitting work from the same door-closed, head-count routine.