We know that pointers give beginning location of Payload in SONET/SDH frame.So how to get end of payload info?

As we know that  the beginning location of frame is given by H1H2 value and also that H1+H2 indicates the offset (in bytes) from H3 to the SPE (i.e. if 0 then J1 POH byte is immediately after H3 in the row). Similarly end can be found because each SPE has a fixed number of bytes.

In H1+H2, 4 MSBs are New Data Flag, 10 LSBs are actual offset value (0 – 782).

So, When offset=522 the STS-1 SPE is in a single STS-1 frame. And In all other cases the SPE straddles two frames When offset is a multiple of 87, the SPE is rectangular. This all happens with ideal synchronization conditions else H3 byte take cares further.
Some fixed start and end locations for various payloads are:-
Pointer Range: 0-103 (VT1.5 type)
Pointer Range: 0-139 (VT2 type)
Pointer Range: 0-211 (VT3 type)
Pointer Range: 0-427 (VT6 type)
Pointer Range: 0-782 (SPE,STS-1 type)
Pointer Range: 0-2339 (STS-3c type)