A microfluidic device comprises a first substrate (102) made of a first polymer material and a second substrate (104) made of a second polymer material, the first (102) and second (104) substrates having respective bonding surfaces (23, 41), at least one of the bonding surfaces (41) having channel formations (14) so that, when the bonding surfaces (23, 41) are bonded by surface deformation to one another, the bonded first and second substrates (102, 104) and the channel formations (14) form at least part of a microfluidic channel network comprising a plurality of microfluidic channels, wherein one or more indicator pits (11), separate to the channel formations (14) defining the microfluidic channel network, are formed in at least one of the bonding surfaces (23, 41), so that surface deformation caused by the bonding process causes a change of configuration of the one or more indicator pits (11).