The more popular, famous tourist attractions (often the Mosque-e-Jhome, Friday Mosque) don't even notice tour-groups wandering around. (Shoes are okay on paved areas, not on carpets or prayer areas.)
I've never had any problem at other smaller, more-active local mosques either. I simply quietly and respectfully enter (removing shoes where and if appropriate) and walk or sit quietly to the side. Discrete photos are possible; just be sensitive and aware.
If not sure, simply talk to someone local. Likely as not, they will be proud to show you around "their" mosque -- even the best places to take photos, or some special interior room. If someone appears to be a caretaker, going out of his way to show you special service or access, a tip may be appropriate.