By the number of terminals

DC motor having two input terminals -ve and +ve. where as AC motors are 3 input  terminals (R Y B).

http://www.reliance.com/prodserv/motgen/b7096.htm

Kindly observe this :) 

Correct word would be,' D.C and A.C  machines', as can work as motor as well as, generator as needed. Terminal box, Stator and Rotor are the main parts, to differentiate, the machines visually.

Terminal box : Of A.C  induction machine up to around 5.00 H.P (delta connected), there would be three for line 1,2,3 and one for ground total four, terminals would be in box.For higher rating six terminals( R1,2.Y1,2.B1,2.) two for each winding plus neutral, total seven(in slip ring motor 3 more. from brush, total ten). For synchronous machines rotor field is supplied by slip rings, hence total nine.

D.C machines are having two armature(A1,A2) and two field(F1,F2) total four terminals in the box.

Rotor : A.C induction machine rotor would be cage or wound. A.C synchronous machines rotor would be round or salient pole, with field winding(F1,F2), and mounted on bearings.For D.C machines rotor(armature) is wound rotor, supplied via commutator and brushes, the rated D.C, through terminal marked A1,A2

Stator : Having fins on surface for A.C machines stator, and is wound(produced rotating magnetic field,and loss IsqrRs in stator)), For D.C machines poles are mounted on cylindrical stator, and are supplied by field(F1,F2) terminals, generally having no fins(as loss IsqrRa, in rotor), on surface.

If the machine is above about 1 hp and

(i) has 6 similar size terminals, then it is a 3-phase induction motor. But it may be either a cage rotor type (6 stator terminals for star-delta starter) or a wound rotor type (3 for stator and 3 for rotor). A visual inspection for the 3 slip rings will confirm.

(ii) has 6 terminals with 4 same size and two smaller, then it is a DC compound motor (2 for armature, 2 for series field with inter-pole and 2 smaller for shunt field). Look for the commutator & brushes and salient field system to confirm.

(iii) has 5 terminals, then it is a AC synchronous motor (3 for stator and 2 for field). Look for 2 slip rings for rotor dc supply.

(iii) has 4 terminals, then it may be a DC motor, in which case look for the commutator & brushes to confirm. Otherwise, it may be a 1-phase alternator (AC generator) in which case there will be 2 slip rings on rotor.

(iv) has 3 terminals, then it is a AC machine, either 3-phase cage rotor induction motor or 3-phase PM synchronous machine. Look into rotor to differentiate.

If the machine is smaller than about 1hp and

(i) has 6 terminals with 4 of same size and 2 smaller, then it is likely to be a 1-phase alternator (2 for armature, 2 for series boosting field and 2 smaller for shunt field). In such case, the armature is on rotor and field system is on stator using salient construction. Look also for 2 slip rings.

(ii) has 4 terminals, then it is a DC machine (2 for armature and 2 for field). Look for the commutator & brushes. Otherwise it may be a simple 1-phase alternator. Look for 2 slip rings on rotor.

(iii) has 3 terminals, then it is a AC machine, either 3-phase cage rotor induction motor or 3-phase PM synchronous machine. Look into rotor construction to differentiate.

(iv) has 2 terminals, then it may be either a 1-phase AC machine or a DC machine. Look for the stator core construction and rotor. If there is no commutator, then it is a AC motor. If there is a commutator and brushes, it may be either a DC motor or an AC commutator motor (Universal motor). However, if the stator core is also laminated in the presence of commutator, then it is likely to be a AC commutator motor. Solid stator core with commutator implies a DC motor.

Note that there are some old types of 3-phase AC commutator motor still in operation, which are known as Schrage motor. Hence presence of commutator does not guarantee DC machine. Also note that all 1-phase AC motor do not use capacitors. Hope you find the elimination process above logical & useful.

DC Machines are often built with adjustable poles in the stator. So if you find bolt heads distributed in pairs on the circumferendce of the stator, it will be a DC machine. Larger DC Machine also have terminals for auxiliary poles an compensation winding, so there can be up to 6 terminals for the armature current and two for the excitation currents. The terminals should be marked according to international standards with Capitals A..F.

you can see the differences in these pictures : 

two steps:

1) check the three phase and an exciter circuit. if yes it is AC

2) check the brushes in DC motors (some of them are brushless but most of them have it)

Some of my friends are implying that presence of brushes indicates DC machine. But they are wrong. AC commutator motors, which are a modified version of the series motor, use brushes and a commutator, and are used as power tool drives (like drills, grinders, cutters, etc) all over the construction site. In fact it is also used in domestic mixers, whippers, food processors, etc. The reason for their popularity in fractional hp range lies in their ability to generate high starting torque on AC supply.

In three phase you can know by checking its three

phase connections at the terminal box. There are three or

four wires in AC motor, while DC motor have two input terminals ( marked as + and -).

you can check the terminal box ( 3 for AC, 2 for DC)  or the motor nameplate.

Simply you can check the casing of the motor. If there is a ventilation so this mean that it is AC motor however if not so it is DC motor. 

By its controller inverter circuit for ac machines and rectifier for DC machines 

1. check the terminal box ( 3 for AC, 4 for DC and has 2 terminals, then it may be either a 1-phase AC machine or a DC machine) .

2. Check if commutator is present than its a DC motor.

By the number of terminals